Active galaxy 4U 1344-60: did the relativistic line disappear?. (arXiv:1209.4068v1 [astro-ph.HE]):
X-ray bright active galactic nuclei represent a unique astrophysical
laboratory for studying accretion physics around super-massive black holes. 4U
1344-60 is a bright Seyfert galaxy which revealed relativistic reflection
features in the archival XMM-Newton observation. We present the spectroscopic
results of new data obtained with the Suzaku satellite and compare them with
the previous XMM-Newton observation. The X-ray continuum of 4U 1344-60 can be
well described by a power-law component with the photon index ~ 1.7 modified by
a fully and a partially covering local absorbers. We measured a substantial
decrease of the fraction of the partially absorbed radiation from around 45% in
the XMM-Newton observation to less than 10% in the Suzaku observation while the
power-law slope remains constant within uncertainties. The iron line in the
Suzaku spectrum is relatively narrow, $\sigma=(0.08 \pm 0.02)$ keV, without any
suggestion for relativistic broadening. Regarding this, we interpret the iron
line in the archival XMM-Newton spectrum as a narrow line of the same width
plus an additional red-shifted emission around 6.1 keV. No evidence of the
relativistic reflection is present in the Suzaku spectra. The detected
red-shifted iron line during the XMM-Newton observation could be a temporary
feature either due to locally enhanced emission or decreased ionisation in the
innermost accretion flow.
Monday, September 24, 2012
Measuring the Ultimate Mass of Galaxy Clusters: Redshifts and Mass Profiles from the Hectospec Cluster Survey (HeCS). (arXiv:1209.3786v1 [astro-ph.CO])
Measuring the Ultimate Mass of Galaxy Clusters: Redshifts and Mass Profiles from the Hectospec Cluster Survey (HeCS). (arXiv:1209.3786v1 [astro-ph.CO]):
The infall regions of galaxy clusters represent the largest gravitationally
bound structures in a $\Lambda$CDM universe. Measuring cluster mass profiles
into the infall regions provides an estimate of the ultimate mass of these
haloes. We use the caustic technique to measure cluster mass profiles from
galaxy redshifts obtained with the Hectospec Cluster Survey (HeCS), an
extensive spectroscopic survey of galaxy clusters with MMT/Hectospec. We survey
58 clusters selected by X-ray flux at 0.1${body}lt;$z$<$0.3. The survey includes
21,314 unique MMT/Hectospec redshifts for individual galaxies; 10,275 of these
galaxies are cluster members. For each cluster we acquired high signal-to-noise
spectra for $\sim 200$ cluster members and a comparable number of
foreground/background galaxies. The cluster members trace out infall patterns
around the clusters. The members define a very narrow red sequence. The
velocity dispersions decline with radius; we demonstrate that the determination
of the velocity dispersion is insensitive to the inclusion of bluer members (a
small fraction of the cluster population). We apply the caustic technique to
define membership and estimate the mass profiles to large radii. The ultimate
halo mass of clusters (the mass that remains bound in the far future of a
$\Lambda$CDM universe) is on average (1.99$\pm$0.11)$M_{200}$, a new
observational cosmological test in essential agreement with simulations. Summed
profiles binned in $M_{200}$ and in $L_X$ demonstrate that the predicted NFW
form of the density profile is a remarkably good representation of the data in
agreement with weak lensing results extending to large radius. The
concentration of these summed profiles is also consistent with theoretical
predictions.
The infall regions of galaxy clusters represent the largest gravitationally
bound structures in a $\Lambda$CDM universe. Measuring cluster mass profiles
into the infall regions provides an estimate of the ultimate mass of these
haloes. We use the caustic technique to measure cluster mass profiles from
galaxy redshifts obtained with the Hectospec Cluster Survey (HeCS), an
extensive spectroscopic survey of galaxy clusters with MMT/Hectospec. We survey
58 clusters selected by X-ray flux at 0.1${body}lt;$z$<$0.3. The survey includes
21,314 unique MMT/Hectospec redshifts for individual galaxies; 10,275 of these
galaxies are cluster members. For each cluster we acquired high signal-to-noise
spectra for $\sim 200$ cluster members and a comparable number of
foreground/background galaxies. The cluster members trace out infall patterns
around the clusters. The members define a very narrow red sequence. The
velocity dispersions decline with radius; we demonstrate that the determination
of the velocity dispersion is insensitive to the inclusion of bluer members (a
small fraction of the cluster population). We apply the caustic technique to
define membership and estimate the mass profiles to large radii. The ultimate
halo mass of clusters (the mass that remains bound in the far future of a
$\Lambda$CDM universe) is on average (1.99$\pm$0.11)$M_{200}$, a new
observational cosmological test in essential agreement with simulations. Summed
profiles binned in $M_{200}$ and in $L_X$ demonstrate that the predicted NFW
form of the density profile is a remarkably good representation of the data in
agreement with weak lensing results extending to large radius. The
concentration of these summed profiles is also consistent with theoretical
predictions.
Self-consistent spectra from radiative GRMHD simulations of accretion onto Sgr A*. (arXiv:1209.4599v1 [astro-ph.HE])
Self-consistent spectra from radiative GRMHD simulations of accretion onto Sgr A*. (arXiv:1209.4599v1 [astro-ph.HE]):
We present the first spectral energy distributions produced self-consistently
by 2.5D general relativistic magneto-hydrodynamical (GRMHD) numerical
simulations, where radiative cooling is included in the dynamical calculation.
As a case study, we focus on the accretion flow around the supermassive black
hole in the Galactic Centre, Sagittarius A* (Sgr A*), which has the best
constrained physical parameters. We compare the simulated spectra to the
observational data of Sgr A* and explore the parameter space of our model to
determine the effect of changing the initial magnetic field configuration, ion
to electron temperature ratio T_i/T_e and the target accretion rate. We find
the best description of the data for a mass accretion rate of ~ 1e-9 Msun/yr,
and rapid spin (0.7 < a_* < 0.9). The submillimeter peak flux seems largely
independent of initial conditions, while the higher energies can be very
sensitive to the initial magnetic field configuration. Finally, we also discuss
flaring features observed in some simulations, that may be due to artifacts of
the 2D configuration.
We present the first spectral energy distributions produced self-consistently
by 2.5D general relativistic magneto-hydrodynamical (GRMHD) numerical
simulations, where radiative cooling is included in the dynamical calculation.
As a case study, we focus on the accretion flow around the supermassive black
hole in the Galactic Centre, Sagittarius A* (Sgr A*), which has the best
constrained physical parameters. We compare the simulated spectra to the
observational data of Sgr A* and explore the parameter space of our model to
determine the effect of changing the initial magnetic field configuration, ion
to electron temperature ratio T_i/T_e and the target accretion rate. We find
the best description of the data for a mass accretion rate of ~ 1e-9 Msun/yr,
and rapid spin (0.7 < a_* < 0.9). The submillimeter peak flux seems largely
independent of initial conditions, while the higher energies can be very
sensitive to the initial magnetic field configuration. Finally, we also discuss
flaring features observed in some simulations, that may be due to artifacts of
the 2D configuration.
A massive bubble of extremely metal poor gas around a collapsing Ly-alpha blob at z=2.54. (arXiv:1209.4676v1 [astro-ph.CO])
A massive bubble of extremely metal poor gas around a collapsing Ly-alpha blob at z=2.54. (arXiv:1209.4676v1 [astro-ph.CO]):
Using long-slit optical spectroscopy obtained at the 10.4 m Gran Telescopio
Canarias, we have examined the gaseous environment of the radio-loud quasar TXS
1436+157 (z=2.54), previously known to be associated with a large Ly-alpha
nebula and a spatially extended Ly-alpha-absorbing structure. From the Ly-alpha
nebula we measure kinematic properties consistent with infall at a rate of
about 10-100 M./yr - more than sufficient to power a quasar at the top of the
luminosity function. The absorbing structure lies outside of the Ly-alpha
nebula, at a radius of >40 kpc from the quasar. Against the bright unresolved
continuum and line emission from the quasar, we detect in absorption the NV
1239,1241, CIV 1548,1551 and SiIV 1394,1403 doublets, with no unambiguous
detection of absorption lines from any low-ionization species of metal. The
metal column densities, taken together with the HI column density measurement
from the literature, indicate that the absorbing gas is predominantly ionized
by the quasar, has a mass of hydrogen of >1.6 x 10E11 M., a gas density of <18
per cubic cm, a line of sight thickness of >18 pc, and a covering factor
approaching unity. While this absorbing structure is clearly not composed of
pristine gas, it has an extremely low metallicity, with ionization models
providing a 3-sigma limit of 12+log(O/H)<7.3. To explain these results, we
discuss a scenario involving starburst-driven super-bubbles and the creation of
infalling filaments of cold gas which fuel/trigger the quasar. We also discuss
the possibility of detecting large-scale absorbers such as this in emission
when illuminated by a powerful quasar.
Using long-slit optical spectroscopy obtained at the 10.4 m Gran Telescopio
Canarias, we have examined the gaseous environment of the radio-loud quasar TXS
1436+157 (z=2.54), previously known to be associated with a large Ly-alpha
nebula and a spatially extended Ly-alpha-absorbing structure. From the Ly-alpha
nebula we measure kinematic properties consistent with infall at a rate of
about 10-100 M./yr - more than sufficient to power a quasar at the top of the
luminosity function. The absorbing structure lies outside of the Ly-alpha
nebula, at a radius of >40 kpc from the quasar. Against the bright unresolved
continuum and line emission from the quasar, we detect in absorption the NV
1239,1241, CIV 1548,1551 and SiIV 1394,1403 doublets, with no unambiguous
detection of absorption lines from any low-ionization species of metal. The
metal column densities, taken together with the HI column density measurement
from the literature, indicate that the absorbing gas is predominantly ionized
by the quasar, has a mass of hydrogen of >1.6 x 10E11 M., a gas density of <18
per cubic cm, a line of sight thickness of >18 pc, and a covering factor
approaching unity. While this absorbing structure is clearly not composed of
pristine gas, it has an extremely low metallicity, with ionization models
providing a 3-sigma limit of 12+log(O/H)<7.3. To explain these results, we
discuss a scenario involving starburst-driven super-bubbles and the creation of
infalling filaments of cold gas which fuel/trigger the quasar. We also discuss
the possibility of detecting large-scale absorbers such as this in emission
when illuminated by a powerful quasar.
X-ray observations of the merging cluster CIZA J2242.8+5301. (arXiv:1209.4642v1 [astro-ph.CO])
X-ray observations of the merging cluster CIZA J2242.8+5301. (arXiv:1209.4642v1 [astro-ph.CO]):
Multiwavelength studies of radio relics at merger shocks set powerful
constraints on the relics origin and formation mechanism. However, for X-ray
observations, a main difficulty is represented by the low X-ray surface
brightness far out in the cluster outskirts, where relics are typically found.
Here, we present XMM-Newton results from a 130-ks observation of CIZA
J2242.8+5301, a cluster at z=0.19 that hosts a double radio relic. We focus on
the well-defined northern relic. There is a difference of ~55% between the
temperature we measure behind the relic, and the temperature measured with
Suzaku. We analyse the reasons for this large discrepancy, and discuss the
possibility of reliably measuring the temperature beyond the northern relic.
Multiwavelength studies of radio relics at merger shocks set powerful
constraints on the relics origin and formation mechanism. However, for X-ray
observations, a main difficulty is represented by the low X-ray surface
brightness far out in the cluster outskirts, where relics are typically found.
Here, we present XMM-Newton results from a 130-ks observation of CIZA
J2242.8+5301, a cluster at z=0.19 that hosts a double radio relic. We focus on
the well-defined northern relic. There is a difference of ~55% between the
temperature we measure behind the relic, and the temperature measured with
Suzaku. We analyse the reasons for this large discrepancy, and discuss the
possibility of reliably measuring the temperature beyond the northern relic.
Ionized Gas in the First 10 Kiloparsecs of the Interstellar Galactic Halo: Metal Ion Fractions. (arXiv:1209.4640v1 [astro-ph.GA])
Ionized Gas in the First 10 Kiloparsecs of the Interstellar Galactic Halo: Metal Ion Fractions. (arXiv:1209.4640v1 [astro-ph.GA]):
We present direct measures of the ionization fractions of several sulfur ions
in the Galactic warm ionized medium (WIM). We obtained high resolution
ultraviolet absorption line spectroscopy of post-asymptotic giant branch stars
in the the globular clusters Messier 3 [(l,b)=(42.2, +78.7); d=10.2 kpc, z=10.0
kpc] and Messier 5 [(l,b)=(3.9, +46.8); d=7.5 kpc, z = +5.3 kpc] with the
Hubble Space Telescope and Far Ultraviolet Spectroscopic Explorer to measure,
or place limits on, the column densities of S I, S II, S III, S IV, S VI, and H
I. These clusters also house millisecond pulsars, whose dispersion measures
give an electron column density from which we infer the H II column in these
directions. We find fractions of S+2 in the WIM for the M 3 and M 5 sight lines
x(S+2) = N(S+2)/N(S) = 0.33+/-0.07 and 0.47+/-0.09, respectively, with
variations perhaps related to location. With negligible quantities of the
higher ionization states, we conclude S+ and S+2 account for all of the S in
the WIM. We extend the methodology to study the ion fractions in the warm and
hot ionized gas of the Milky Way, including the high ions Si+3, C+3, N+4, and
O+5. The vast majority of the Galactic ionized gas is warm (T ~ 10^4 K) and
photoionized (the WIM) or very hot (T > 4x10^5 K) and collisionally ionized.
The common tracer of ionized gas beyond the Milky Way, O+5, traces <1% of the
total ionized gas mass of the Milky Way.
We present direct measures of the ionization fractions of several sulfur ions
in the Galactic warm ionized medium (WIM). We obtained high resolution
ultraviolet absorption line spectroscopy of post-asymptotic giant branch stars
in the the globular clusters Messier 3 [(l,b)=(42.2, +78.7); d=10.2 kpc, z=10.0
kpc] and Messier 5 [(l,b)=(3.9, +46.8); d=7.5 kpc, z = +5.3 kpc] with the
Hubble Space Telescope and Far Ultraviolet Spectroscopic Explorer to measure,
or place limits on, the column densities of S I, S II, S III, S IV, S VI, and H
I. These clusters also house millisecond pulsars, whose dispersion measures
give an electron column density from which we infer the H II column in these
directions. We find fractions of S+2 in the WIM for the M 3 and M 5 sight lines
x(S+2) = N(S+2)/N(S) = 0.33+/-0.07 and 0.47+/-0.09, respectively, with
variations perhaps related to location. With negligible quantities of the
higher ionization states, we conclude S+ and S+2 account for all of the S in
the WIM. We extend the methodology to study the ion fractions in the warm and
hot ionized gas of the Milky Way, including the high ions Si+3, C+3, N+4, and
O+5. The vast majority of the Galactic ionized gas is warm (T ~ 10^4 K) and
photoionized (the WIM) or very hot (T > 4x10^5 K) and collisionally ionized.
The common tracer of ionized gas beyond the Milky Way, O+5, traces <1% of the
total ionized gas mass of the Milky Way.
Thursday, September 20, 2012
The high-redshift (z>3) AGN population in the 4 Ms Chandra Deep Field South. (arXiv:1209.4193v1 [astro-ph.CO])
The high-redshift (z>3) AGN population in the 4 Ms Chandra Deep Field South. (arXiv:1209.4193v1 [astro-ph.CO]):
We present results from a spectral analysis of a sample of high-redshift
(z>3) X-ray selected AGN in the 4 Ms Chandra Deep Field South (CDF-S), the
deepest X-ray survey to date. The sample is selected using the most recent
spectroscopic and photometric information available in this field. It consists
of 34 sources with median redshift z=3.7, 80 median net counts in the 0.5-7 keV
band and median rest-frame absorption-corrected luminosity $L_{2-10
\rmn{keV}}\approx1.5\times10^{44}\rmn{erg} \rmn{s^{-1}}$. Spectral analysis for
the full sample is presented and the intrinsic column density distribution,
corrected for observational biases using spectral simulations, is compared with
the expectations of X-ray background (XRB) synthesis models. We find that
$\approx57$ per cent of the sources are highly obscured
($N_H>10^{23}\rmn{cm^{-2}}$). Source number counts in the $0.5-2\rmn{keV}$ band
down to flux $F_{0.5-2 \rmn{keV}}\approx4\times10^{-17}\rmn{erg}
\rmn{s^{-1}cm^{-2}}$ are also presented. Our results are consistent with a
decline of the AGN space density at z>3 and suggest that, at those redshifts,
the AGN obscured fraction is in agreement with the expectations of XRB
synthesis models.
We present results from a spectral analysis of a sample of high-redshift
(z>3) X-ray selected AGN in the 4 Ms Chandra Deep Field South (CDF-S), the
deepest X-ray survey to date. The sample is selected using the most recent
spectroscopic and photometric information available in this field. It consists
of 34 sources with median redshift z=3.7, 80 median net counts in the 0.5-7 keV
band and median rest-frame absorption-corrected luminosity $L_{2-10
\rmn{keV}}\approx1.5\times10^{44}\rmn{erg} \rmn{s^{-1}}$. Spectral analysis for
the full sample is presented and the intrinsic column density distribution,
corrected for observational biases using spectral simulations, is compared with
the expectations of X-ray background (XRB) synthesis models. We find that
$\approx57$ per cent of the sources are highly obscured
($N_H>10^{23}\rmn{cm^{-2}}$). Source number counts in the $0.5-2\rmn{keV}$ band
down to flux $F_{0.5-2 \rmn{keV}}\approx4\times10^{-17}\rmn{erg}
\rmn{s^{-1}cm^{-2}}$ are also presented. Our results are consistent with a
decline of the AGN space density at z>3 and suggest that, at those redshifts,
the AGN obscured fraction is in agreement with the expectations of XRB
synthesis models.
On the Cluster Physics of Sunyaev-Zel'dovich and X-ray Surveys III: Measurement Biases and Cosmological Evolution of Gas and Stellar Mass Fractions. (arXiv:1209.4082v1 [astro-ph.CO])
On the Cluster Physics of Sunyaev-Zel'dovich and X-ray Surveys III: Measurement Biases and Cosmological Evolution of Gas and Stellar Mass Fractions. (arXiv:1209.4082v1 [astro-ph.CO]):
Gas masses tightly correlate with the virial masses of galaxy clusters,
allowing for a precise determination of cosmological parameters by means of
large-scale X-ray surveys. However, according to recent Suzaku X-ray
measurements, gas mass fractions, f_gas, appear to be considerably larger than
the cosmic mean at the virial radius, R_200, questioning the accuracy of the
cosmological parameter estimations. Here, we use a large suite of cosmological
hydrodynamical simulations to study measurement biases of f_gas. We employ
different variants of simulated physics, including radiative gas physics, star
formation, and thermal feedback by active galactic nuclei. Computing the mass
profiles in 48 angular cones, whose footprints partition the sphere, we find
anisotropic gas and total mass distributions that imply an angular variance of
f_gas at the level of 30%. This anisotropic distribution originates from the
recent formation epoch of clusters and from the strong internal
baryon-to-dark-matter density bias. In the most extreme cones, f_gas can be
biased high by a factor of two at R_200 in massive clusters, thereby providing
a potential explanation for high f_gas measurements by Suzaku. While projection
lowers this factor, there are other measurement biases that may (partially)
compensate. We find that at R_200, f_gas is biased high by 20% when assuming
hydrostatic equilibrium masses, i.e., neglecting the kinetic pressure, and by
another ~10-20% due to the presence of density clumping. At larger radii, both
measurement biases increase dramatically. While the cluster sample variance of
the true f_gas decreases to a level of 5% at R_200, the sample variance that
includes both measurement biases remains fairly constant at the level of
10-20%. The constant redshift evolution of f_gas within R_500 for massive
clusters is encouraging for using gas masses to derive cosmological parameters.
Gas masses tightly correlate with the virial masses of galaxy clusters,
allowing for a precise determination of cosmological parameters by means of
large-scale X-ray surveys. However, according to recent Suzaku X-ray
measurements, gas mass fractions, f_gas, appear to be considerably larger than
the cosmic mean at the virial radius, R_200, questioning the accuracy of the
cosmological parameter estimations. Here, we use a large suite of cosmological
hydrodynamical simulations to study measurement biases of f_gas. We employ
different variants of simulated physics, including radiative gas physics, star
formation, and thermal feedback by active galactic nuclei. Computing the mass
profiles in 48 angular cones, whose footprints partition the sphere, we find
anisotropic gas and total mass distributions that imply an angular variance of
f_gas at the level of 30%. This anisotropic distribution originates from the
recent formation epoch of clusters and from the strong internal
baryon-to-dark-matter density bias. In the most extreme cones, f_gas can be
biased high by a factor of two at R_200 in massive clusters, thereby providing
a potential explanation for high f_gas measurements by Suzaku. While projection
lowers this factor, there are other measurement biases that may (partially)
compensate. We find that at R_200, f_gas is biased high by 20% when assuming
hydrostatic equilibrium masses, i.e., neglecting the kinetic pressure, and by
another ~10-20% due to the presence of density clumping. At larger radii, both
measurement biases increase dramatically. While the cluster sample variance of
the true f_gas decreases to a level of 5% at R_200, the sample variance that
includes both measurement biases remains fairly constant at the level of
10-20%. The constant redshift evolution of f_gas within R_500 for massive
clusters is encouraging for using gas masses to derive cosmological parameters.
Wednesday, September 19, 2012
The r-mode instability in strange stars with a crystalline crust. (arXiv:1209.4343v1 [nucl-th])
The r-mode instability in strange stars with a crystalline crust. (arXiv:1209.4343v1 [nucl-th]):
The r-mode instability, believed to limit the rotation speed of compact
stars, can provide empirical confirmation for the existence of stable
deconfined phases of quark matter that are predicted by weak coupling
calculations in Quantum Chromodynamics. We construct a model for strange quark
stars as heavy as 2 solar masses that are made of superconducting quark matter
in the bulk and a thin crystalline quark matter crust. This crystalline quark
crust is sufficiently robust to withstand r-mode heating and viscous rubbing
for realistic mode amplitudes O(10^{-2}), unlike a crust made of neutron-rich
nuclei. The dissipation provided by viscous rubbing at the core-crust boundary
is both necessary and sufficient to obtain stable rotation speeds that are
consistent with the majority of rapidly spinning pulsars in low mass X-ray
binaries. Our analysis implies that while bare strange stars are ruled out by
the existence of rapidly spinning pulsars, a strange star with a quark matter
crust is a distinct possibility.
The r-mode instability, believed to limit the rotation speed of compact
stars, can provide empirical confirmation for the existence of stable
deconfined phases of quark matter that are predicted by weak coupling
calculations in Quantum Chromodynamics. We construct a model for strange quark
stars as heavy as 2 solar masses that are made of superconducting quark matter
in the bulk and a thin crystalline quark matter crust. This crystalline quark
crust is sufficiently robust to withstand r-mode heating and viscous rubbing
for realistic mode amplitudes O(10^{-2}), unlike a crust made of neutron-rich
nuclei. The dissipation provided by viscous rubbing at the core-crust boundary
is both necessary and sufficient to obtain stable rotation speeds that are
consistent with the majority of rapidly spinning pulsars in low mass X-ray
binaries. Our analysis implies that while bare strange stars are ruled out by
the existence of rapidly spinning pulsars, a strange star with a quark matter
crust is a distinct possibility.
Warm-hot gas in groups and galaxies toward H2356-309. (arXiv:1209.4080v1 [astro-ph.CO])
Warm-hot gas in groups and galaxies toward H2356-309. (arXiv:1209.4080v1 [astro-ph.CO]):
We present a detailed analysis of the galaxy and group distributions around
three reported X-ray absorption line systems in the spectrum of the quasar
H2356-309. Previous studies associated these absorbers with known large-scale
galaxy structures (i.e., walls and filaments) along the line of sight. Such
absorption lines typically trace 10^{5-7} K gas, and may be evidence of the
elusive warm-hot intergalactic medium (WHIM) thought to harbor the bulk of the
low-redshift "missing baryons;" alternatively, they may be linked to individual
galaxies or groups in the filaments. Here we combine existing galaxy survey
data with new, highly complete multi-object Magellan spectroscopy to
investigate the detailed galaxy distribution near each absorber. All of these
three absorption systems nominally lie within the virial radii of nearby
galaxies and/or groups, and could therefore arise in these virialized
structures rather than (or in addition to) the WHIM. However, we find no
additional galaxies near a fourth "void" absorber recently reported by
Zappacosta et al., suggesting that this system may indeed trace gas
unassociated with any individual halo. We therefore conclude that most X-ray
absorbers are coincident with galaxy and/or group environments, though some
could still trace the large-scale filamentary WHIM gas predicted by
simulations.
We present a detailed analysis of the galaxy and group distributions around
three reported X-ray absorption line systems in the spectrum of the quasar
H2356-309. Previous studies associated these absorbers with known large-scale
galaxy structures (i.e., walls and filaments) along the line of sight. Such
absorption lines typically trace 10^{5-7} K gas, and may be evidence of the
elusive warm-hot intergalactic medium (WHIM) thought to harbor the bulk of the
low-redshift "missing baryons;" alternatively, they may be linked to individual
galaxies or groups in the filaments. Here we combine existing galaxy survey
data with new, highly complete multi-object Magellan spectroscopy to
investigate the detailed galaxy distribution near each absorber. All of these
three absorption systems nominally lie within the virial radii of nearby
galaxies and/or groups, and could therefore arise in these virialized
structures rather than (or in addition to) the WHIM. However, we find no
additional galaxies near a fourth "void" absorber recently reported by
Zappacosta et al., suggesting that this system may indeed trace gas
unassociated with any individual halo. We therefore conclude that most X-ray
absorbers are coincident with galaxy and/or group environments, though some
could still trace the large-scale filamentary WHIM gas predicted by
simulations.
Tuesday, September 18, 2012
A Broad Iron Line in LMC X-1. (arXiv:1209.3269v1 [astro-ph.HE])
A Broad Iron Line in LMC X-1. (arXiv:1209.3269v1 [astro-ph.HE]):
We present results from a deep Suzaku observation of the black hole in LMC
X-1, supplemented by coincident monitoring with RXTE. We identify broad
relativistic reflection features in a soft disc-dominated spectrum. A strong
and variable power-law component of emission is present which we use to
demonstrate that enhanced Comptonisation strengthens disc reflection. We
constrain the spin parameter of the black hole by modelling LMC X-1's broad
reflection features. For our primary and most comprehensive spectral model, we
obtain a high value for the spin: a* = 0.97(+0.01,-0.13) (68 per cent
confidence). However, by additionally considering two alternate models as a
measure of our systematic uncertainty, we obtain a broader constraint: a* =
0.97(+0.02,-0.25). Both of these spin values are entirely consistent with a
previous estimate of spin obtained using the continuum-fitting method. At 99
per cent confidence, the reflection features require a* > 0.2. In addition to
modelling the relativistically broadened reflection, we also model a sharp and
prominent reflection component that provides strong evidence for substantial
reprocessing in the wind of the massive companion. We infer that this wind
sustains the ionisation cone surrounding the binary system; this hypothesis
naturally produces appropriate and consistent mass, time, and length scales for
the cone structure.
We present results from a deep Suzaku observation of the black hole in LMC
X-1, supplemented by coincident monitoring with RXTE. We identify broad
relativistic reflection features in a soft disc-dominated spectrum. A strong
and variable power-law component of emission is present which we use to
demonstrate that enhanced Comptonisation strengthens disc reflection. We
constrain the spin parameter of the black hole by modelling LMC X-1's broad
reflection features. For our primary and most comprehensive spectral model, we
obtain a high value for the spin: a* = 0.97(+0.01,-0.13) (68 per cent
confidence). However, by additionally considering two alternate models as a
measure of our systematic uncertainty, we obtain a broader constraint: a* =
0.97(+0.02,-0.25). Both of these spin values are entirely consistent with a
previous estimate of spin obtained using the continuum-fitting method. At 99
per cent confidence, the reflection features require a* > 0.2. In addition to
modelling the relativistically broadened reflection, we also model a sharp and
prominent reflection component that provides strong evidence for substantial
reprocessing in the wind of the massive companion. We infer that this wind
sustains the ionisation cone surrounding the binary system; this hypothesis
naturally produces appropriate and consistent mass, time, and length scales for
the cone structure.
Massive Stars: Key to Solving the Cosmic Puzzle. (arXiv:1209.3199v1 [astro-ph.SR])
Massive Stars: Key to Solving the Cosmic Puzzle. (arXiv:1209.3199v1 [astro-ph.SR]):
We describe observations in the nearby universe (<100 Mpc) with a 10-m or
larger space-based telescope having imaging and spectral capabilities in the
range 912-9000 \AA that would enable advances in the fields of massive stars,
young populations, and star-forming galaxies, that are essential for achieving
the Cosmic Origins Program objectives i) how are the chemical elements
distributed in galaxies and dispersed in the circumgalactic and intergalactic
medium; and ii) when did the first stars in the universe form, and how did they
influence their environments. We stress the importance of observing hundreds of
massive stars and their descendants individually, which will make it possible
to separate the many competing factors that influence the observed properties
of these systems (mass, composition, convection, mass-loss, rotation rate,
binarity, magnetic fields, and cluster mass).
We describe observations in the nearby universe (<100 Mpc) with a 10-m or
larger space-based telescope having imaging and spectral capabilities in the
range 912-9000 \AA that would enable advances in the fields of massive stars,
young populations, and star-forming galaxies, that are essential for achieving
the Cosmic Origins Program objectives i) how are the chemical elements
distributed in galaxies and dispersed in the circumgalactic and intergalactic
medium; and ii) when did the first stars in the universe form, and how did they
influence their environments. We stress the importance of observing hundreds of
massive stars and their descendants individually, which will make it possible
to separate the many competing factors that influence the observed properties
of these systems (mass, composition, convection, mass-loss, rotation rate,
binarity, magnetic fields, and cluster mass).
Synergistic Astrophysics in the Ultraviolet using Active Galactic Nuclei. (arXiv:1209.3196v1 [astro-ph.CO])
Synergistic Astrophysics in the Ultraviolet using Active Galactic Nuclei. (arXiv:1209.3196v1 [astro-ph.CO]):
Observing programs comprising multiple scientific objectives will enhance the
productivity of NASA's next UV/Visible mission. Studying active galactic nuclei
(AGN) is intrinsically important for understanding how black holes accrete
matter, grow through cosmic time, and influence their host galaxies. At the
same time, the bright UV continuum of AGN serves as an ideal background light
source for studying foreground gas in the intergalactic medium (IGM), the
circumgalactic medium (CGM) of individual galaxies, and the interstellar medium
(ISM) and halo of the Milky Way. A well chosen sample of AGN can serve as the
observational backbone for multiple spectroscopic investigations including
quantitative measurements of outflows from AGN, the structure of their
accretion disks, and the mass of the central black hole.
Observing programs comprising multiple scientific objectives will enhance the
productivity of NASA's next UV/Visible mission. Studying active galactic nuclei
(AGN) is intrinsically important for understanding how black holes accrete
matter, grow through cosmic time, and influence their host galaxies. At the
same time, the bright UV continuum of AGN serves as an ideal background light
source for studying foreground gas in the intergalactic medium (IGM), the
circumgalactic medium (CGM) of individual galaxies, and the interstellar medium
(ISM) and halo of the Milky Way. A well chosen sample of AGN can serve as the
observational backbone for multiple spectroscopic investigations including
quantitative measurements of outflows from AGN, the structure of their
accretion disks, and the mass of the central black hole.
Testing General Relativity in the Strong-Field Regime with Observations of Black Holes in the Electromagnetic Spectrum. (arXiv:1209.3024v1 [astro-ph.HE])
Testing General Relativity in the Strong-Field Regime with Observations of Black Holes in the Electromagnetic Spectrum. (arXiv:1209.3024v1 [astro-ph.HE]):
General relativity has been tested by many experiments, which, however,
almost exclusively probe weak spacetime curvatures. In this thesis, I create
two frameworks for testing general relativity in the strong-field regime with
observations of black holes in the electromagnetic spectrum using current or
near-future instruments. In the first part of this thesis, I design tests of
the no-hair theorem, which uniquely characterizes the nature of black holes in
general relativity in terms of their masses and spins and which states that
these compact objects are described by the Kerr metric. I investigate a
quasi-Kerr metric and construct a Kerr-like spacetime, both of which contain an
independent parameter in addition to mass and spin. If the no-hair theorem is
correct, then any deviation from the Kerr metric has to be zero. I show that
already moderate changes of the deviation parameters in either metric lead to
significant modifications of the observed signals. I apply this framework to
the imaging of supermassive black holes using very-long baseline interferometry
as well as to the quasi-periodic variability and relativistically broadened
iron lines observed in both galactic and supermassive black holes. In the
second part of this thesis, I devise a method to test the predicted evaporation
of black holes in Randall-Sundrum-type braneworld gravity through the orbital
evolution of black-hole X-ray binaries and obtain constraints on the size of
the extra dimension from A0620-00 and XTE J1118+480.
General relativity has been tested by many experiments, which, however,
almost exclusively probe weak spacetime curvatures. In this thesis, I create
two frameworks for testing general relativity in the strong-field regime with
observations of black holes in the electromagnetic spectrum using current or
near-future instruments. In the first part of this thesis, I design tests of
the no-hair theorem, which uniquely characterizes the nature of black holes in
general relativity in terms of their masses and spins and which states that
these compact objects are described by the Kerr metric. I investigate a
quasi-Kerr metric and construct a Kerr-like spacetime, both of which contain an
independent parameter in addition to mass and spin. If the no-hair theorem is
correct, then any deviation from the Kerr metric has to be zero. I show that
already moderate changes of the deviation parameters in either metric lead to
significant modifications of the observed signals. I apply this framework to
the imaging of supermassive black holes using very-long baseline interferometry
as well as to the quasi-periodic variability and relativistically broadened
iron lines observed in both galactic and supermassive black holes. In the
second part of this thesis, I devise a method to test the predicted evaporation
of black holes in Randall-Sundrum-type braneworld gravity through the orbital
evolution of black-hole X-ray binaries and obtain constraints on the size of
the extra dimension from A0620-00 and XTE J1118+480.
No clear submillimetre signature of suppressed star formation amongst X-ray luminous AGNs. (arXiv:1209.3016v1 [astro-ph.CO])
No clear submillimetre signature of suppressed star formation amongst X-ray luminous AGNs. (arXiv:1209.3016v1 [astro-ph.CO]):
Many theoretical models require powerful active galactic nuclei (AGNs) to
suppress star formation in distant galaxies and reproduce the observed
properties of today's massive galaxies. A recent study based on Herschel-SPIRE
submillimetre observations claimed to provide direct support for this picture,
reporting a significant decrease in the mean star-formation rates (SFRs) of the
most luminous AGNs (Lx>10^44 erg/s) at z=1-3 in the Chandra Deep Field-North
(CDF-N). In this letter we extend these results using Herschel-SPIRE 250um data
in the COSMOS and CDF-S fields to achieve an order of magnitude improvement in
the number of sources at Lx>10^44 erg/s. On the basis of our analyses, we find
no strong evidence for suppressed star formation in Lx>10^44 erg/s AGNs at
z=1-3. The mean SFRs of the AGNs are constant over the broad X-ray luminosity
range of Lx~10^43-10^45 erg/s (with mean SFRs consistent with typical
star-forming galaxies at z~2; <SFRs>~100-200 Msol/yr). We suggest that the
previous CDF-N results were likely due to low number statistics. We discuss our
results in the context of current theoretical models and suggest that it will
be challenging to see the signature of suppressed star formation simply on the
basis of an X-ray luminosity threshold.
Many theoretical models require powerful active galactic nuclei (AGNs) to
suppress star formation in distant galaxies and reproduce the observed
properties of today's massive galaxies. A recent study based on Herschel-SPIRE
submillimetre observations claimed to provide direct support for this picture,
reporting a significant decrease in the mean star-formation rates (SFRs) of the
most luminous AGNs (Lx>10^44 erg/s) at z=1-3 in the Chandra Deep Field-North
(CDF-N). In this letter we extend these results using Herschel-SPIRE 250um data
in the COSMOS and CDF-S fields to achieve an order of magnitude improvement in
the number of sources at Lx>10^44 erg/s. On the basis of our analyses, we find
no strong evidence for suppressed star formation in Lx>10^44 erg/s AGNs at
z=1-3. The mean SFRs of the AGNs are constant over the broad X-ray luminosity
range of Lx~10^43-10^45 erg/s (with mean SFRs consistent with typical
star-forming galaxies at z~2; <SFRs>~100-200 Msol/yr). We suggest that the
previous CDF-N results were likely due to low number statistics. We discuss our
results in the context of current theoretical models and suggest that it will
be challenging to see the signature of suppressed star formation simply on the
basis of an X-ray luminosity threshold.
A corresponding-state approach to quark-cluster matter. (arXiv:1209.3688v1 [astro-ph.SR])
A corresponding-state approach to quark-cluster matter. (arXiv:1209.3688v1 [astro-ph.SR]):
The state of super-dense matter is essential for us to understand the nature
of pulsars, but the non-perturbative quantum chromodynamics (QCD) makes it very
difficult for direct calculations of the state of cold matter at realistic
baryon number densities inside compact stars. The strong coupling between
quarks might render quarks grouped in clusters, and at high densities but low
temperature, it is conjectured that the quark-cluster matter could be in a
condensed phase as self-bound objects. Nevertheless, supposing that the
quark-clusters could be analogized to inert gases, we apply here the
corresponding-state approach to derive the equation of state of quark-cluster
matter, as was demonstrated for nuclear and neutron-star matter in 1970s.
According to the calculations presented, the quark-cluster stars, which are
composed of quark-cluster matter, could then have high maximum mass that is
consistent with observations and, in turn, further observations of pulsar mass
would also put constraints to the properties of quark-cluster matter. Moreover,
the melting heat during solid-liquid phase conversion and the related
astrophysical consequences are also discussed.
The state of super-dense matter is essential for us to understand the nature
of pulsars, but the non-perturbative quantum chromodynamics (QCD) makes it very
difficult for direct calculations of the state of cold matter at realistic
baryon number densities inside compact stars. The strong coupling between
quarks might render quarks grouped in clusters, and at high densities but low
temperature, it is conjectured that the quark-cluster matter could be in a
condensed phase as self-bound objects. Nevertheless, supposing that the
quark-clusters could be analogized to inert gases, we apply here the
corresponding-state approach to derive the equation of state of quark-cluster
matter, as was demonstrated for nuclear and neutron-star matter in 1970s.
According to the calculations presented, the quark-cluster stars, which are
composed of quark-cluster matter, could then have high maximum mass that is
consistent with observations and, in turn, further observations of pulsar mass
would also put constraints to the properties of quark-cluster matter. Moreover,
the melting heat during solid-liquid phase conversion and the related
astrophysical consequences are also discussed.
Neutron Star Properties in the Chiral Quark-Meson Coupling Model. (arXiv:1209.3360v1 [nucl-th])
Neutron Star Properties in the Chiral Quark-Meson Coupling Model. (arXiv:1209.3360v1 [nucl-th]):
We study the properties of neutron star using the chiral quark-meson coupling
model, in which the quark-quark hyperfine interaction due to the exchanges of
gluon and pion based on chiral symmetry is considered. We also examine the
effects of hyperons and $\Delta$-isobars in a neutron star. Extending the SU(6)
spin-flavor symmetry to more general SU(3) flavor symmetry in the vector-meson
couplings to baryons, the maximum mass of neutron star can reach the recently
observed, massive pulsar mass, $1.97 \pm 0.04 M_{\odot}$. In this calculation,
$\Lambda$ and $\Xi$ are generated in a neutron star, while $\Sigma$ and
$\Delta$-isobars do not appear.
We study the properties of neutron star using the chiral quark-meson coupling
model, in which the quark-quark hyperfine interaction due to the exchanges of
gluon and pion based on chiral symmetry is considered. We also examine the
effects of hyperons and $\Delta$-isobars in a neutron star. Extending the SU(6)
spin-flavor symmetry to more general SU(3) flavor symmetry in the vector-meson
couplings to baryons, the maximum mass of neutron star can reach the recently
observed, massive pulsar mass, $1.97 \pm 0.04 M_{\odot}$. In this calculation,
$\Lambda$ and $\Xi$ are generated in a neutron star, while $\Sigma$ and
$\Delta$-isobars do not appear.
A Fast Flare and Direct Redshift Constraint in Far-UV Spectra of the Blazar S50716+714. (arXiv:1209.3325v1 [astro-ph.HE])
A Fast Flare and Direct Redshift Constraint in Far-UV Spectra of the Blazar S50716+714. (arXiv:1209.3325v1 [astro-ph.HE]):
The BL Lacertae object S50716+714 is one of the most studied blazars on the
sky due to its active variability and brightness in many bands, including VHE
gamma rays. We present here two serendipitous results from recent
far-ultraviolet spectroscopic observations by the Cosmic Origins Spectrograph
onboard the Hubble Space Telescope. First, the blazar increased in flux rapidly
by ~40% (-0.45 mag/h) followed by a slower decline (+0.36 mag/h) to previous
far-UV flux levels during the course of our 7.3 hour HST observations. We model
this flare using asymmetric flare templates and constrain the physical size,
and energetics of the emitting region. Furthermore, the spectral index of the
object softens considerably during the course of the flare from alpha_nu=-1.0
to alpha_nu=-1.4. Second, we constrain the source redshift directly using the
>30 intervening absorption systems. A system at z=0.2315 is detected in Lya,
Lyb, OVI, NV, and CIII and defines the lower bound on the source redshift. No
absorbers are seen in the remaining spectral coverage (0.2315<z_Lya<0.47) and
we set a statistical upper bound of z<0.304 (90% confidence) on the blazar.
This is the first direct redshift limit for this object and is consistent with
literature estimates of z=0.31+-0.08 based on the detection of a host galaxy.
The BL Lacertae object S50716+714 is one of the most studied blazars on the
sky due to its active variability and brightness in many bands, including VHE
gamma rays. We present here two serendipitous results from recent
far-ultraviolet spectroscopic observations by the Cosmic Origins Spectrograph
onboard the Hubble Space Telescope. First, the blazar increased in flux rapidly
by ~40% (-0.45 mag/h) followed by a slower decline (+0.36 mag/h) to previous
far-UV flux levels during the course of our 7.3 hour HST observations. We model
this flare using asymmetric flare templates and constrain the physical size,
and energetics of the emitting region. Furthermore, the spectral index of the
object softens considerably during the course of the flare from alpha_nu=-1.0
to alpha_nu=-1.4. Second, we constrain the source redshift directly using the
>30 intervening absorption systems. A system at z=0.2315 is detected in Lya,
Lyb, OVI, NV, and CIII and defines the lower bound on the source redshift. No
absorbers are seen in the remaining spectral coverage (0.2315<z_Lya<0.47) and
we set a statistical upper bound of z<0.304 (90% confidence) on the blazar.
This is the first direct redshift limit for this object and is consistent with
literature estimates of z=0.31+-0.08 based on the detection of a host galaxy.
Solving the Cooling Flow Problem through Mechanical AGN Feedback. (arXiv:1209.3305v1 [astro-ph.CO])
Solving the Cooling Flow Problem through Mechanical AGN Feedback. (arXiv:1209.3305v1 [astro-ph.CO]):
Unopposed radiative cooling of plasma would lead to the cooling catastrophe,
a massive inflow of condensing gas, manifest in the core of galaxies, groups
and clusters. The last generation X-ray telescopes, Chandra and XMM, have
radically changed our view on baryons, indicating AGN heating as the balancing
counterpart of cooling. This work reviews our extensive investigation on
self-regulated heating. We argue that the mechanical feedback, based on massive
subrelativistic outflows, is the key to solving the cooling flow problem, i.e.
dramatically quenching the cooling rates for several Gyr without destroying the
cool-core structure. Using a modified version of the 3D hydrocode FLASH, we
show that bipolar AGN outflows can further reproduce fundamental observed
features, such as buoyant bubbles, weak shocks, metals dredge- up, and
turbulence. The latter is an essential ingredient to drive nonlinear thermal
instabilities, which cause the formation of extended cold gas, a residual of
the quenched cooling flow and, later, fuel for the feedback engine. Compared to
clusters, groups and galaxies require a gentler mechanical feedback, in order
to avoid catastrophic overheating. We highlight the essential characteristics
for a realistic AGN feedback, with emphasis on observational consistency.
Unopposed radiative cooling of plasma would lead to the cooling catastrophe,
a massive inflow of condensing gas, manifest in the core of galaxies, groups
and clusters. The last generation X-ray telescopes, Chandra and XMM, have
radically changed our view on baryons, indicating AGN heating as the balancing
counterpart of cooling. This work reviews our extensive investigation on
self-regulated heating. We argue that the mechanical feedback, based on massive
subrelativistic outflows, is the key to solving the cooling flow problem, i.e.
dramatically quenching the cooling rates for several Gyr without destroying the
cool-core structure. Using a modified version of the 3D hydrocode FLASH, we
show that bipolar AGN outflows can further reproduce fundamental observed
features, such as buoyant bubbles, weak shocks, metals dredge- up, and
turbulence. The latter is an essential ingredient to drive nonlinear thermal
instabilities, which cause the formation of extended cold gas, a residual of
the quenched cooling flow and, later, fuel for the feedback engine. Compared to
clusters, groups and galaxies require a gentler mechanical feedback, in order
to avoid catastrophic overheating. We highlight the essential characteristics
for a realistic AGN feedback, with emphasis on observational consistency.
Friday, September 14, 2012
Observed Limits on Charge Exchange Contributions to the Diffuse X-ray Background. (arXiv:1209.1657v1 [astro-ph.HE])
Observed Limits on Charge Exchange Contributions to the Diffuse X-ray Background. (arXiv:1209.1657v1 [astro-ph.HE]):
We present a high resolution spectrum of the diffuse X-ray background from
0.1 to 1 keV for a ~1 region of the sky centered at l=90, b=+60 using a
36-pixel array of microcalorimeters flown on a sounding rocket. With an energy
resolution of 11 eV FWHM below 1 keV, the spectrum's observed line ratios help
separate charge exchange contributions originating within the heliosphere from
thermal emission of hot gas in the interstellar medium. The X-ray sensitivity
below 1 keV was reduced by about a factor of four from contamination that
occurred early in the flight, limiting the significance of the results. The
observed centroid of helium-like O VII is 568+2-3 eV at 90% confidence. Since
the centroid expected for thermal emission is 568.4 eV while for charge
exchange is 564.2 eV, thermal emission appears to dominate for this line
complex, consistent with much of the high-latitude O VII emission originating
in 2-3 x 10^6 K gas in the Galactic halo. On the other hand, the observed ratio
of C VI Ly gamma to Ly alpha is 0.3+-0.2. The expected ratios are 0.04 for
thermal emission and 0.24 for charge exchange, indicating that charge exchange
must contribute strongly to this line and therefore potentially to the rest of
the ROSAT R12 band usually associated with 10^6 K emission from the Local Hot
Bubble. The limited statistics of this experiment and systematic uncertainties
due to the contamination require only >32% thermal emission for O VII and >20%
from charge exchange for C VI at the 90% confidence level. An experimental gold
coating on the silicon substrate of the array greatly reduced extraneous
signals induced on nearby pixels from cosmic rays passing through the
substrate, reducing the triggered event rate by a factor of 15 from a previous
flight of the instrument.
We present a high resolution spectrum of the diffuse X-ray background from
0.1 to 1 keV for a ~1 region of the sky centered at l=90, b=+60 using a
36-pixel array of microcalorimeters flown on a sounding rocket. With an energy
resolution of 11 eV FWHM below 1 keV, the spectrum's observed line ratios help
separate charge exchange contributions originating within the heliosphere from
thermal emission of hot gas in the interstellar medium. The X-ray sensitivity
below 1 keV was reduced by about a factor of four from contamination that
occurred early in the flight, limiting the significance of the results. The
observed centroid of helium-like O VII is 568+2-3 eV at 90% confidence. Since
the centroid expected for thermal emission is 568.4 eV while for charge
exchange is 564.2 eV, thermal emission appears to dominate for this line
complex, consistent with much of the high-latitude O VII emission originating
in 2-3 x 10^6 K gas in the Galactic halo. On the other hand, the observed ratio
of C VI Ly gamma to Ly alpha is 0.3+-0.2. The expected ratios are 0.04 for
thermal emission and 0.24 for charge exchange, indicating that charge exchange
must contribute strongly to this line and therefore potentially to the rest of
the ROSAT R12 band usually associated with 10^6 K emission from the Local Hot
Bubble. The limited statistics of this experiment and systematic uncertainties
due to the contamination require only >32% thermal emission for O VII and >20%
from charge exchange for C VI at the 90% confidence level. An experimental gold
coating on the silicon substrate of the array greatly reduced extraneous
signals induced on nearby pixels from cosmic rays passing through the
substrate, reducing the triggered event rate by a factor of 15 from a previous
flight of the instrument.
Accreting SMBHs in the COSMOS field and the connection to their host galaxies. (arXiv:1209.1640v1 [astro-ph.CO])
Accreting SMBHs in the COSMOS field and the connection to their host galaxies. (arXiv:1209.1640v1 [astro-ph.CO]):
Using the wide multi-band photometry available in the COSMOS field we explore
the host galaxy properties of a large sample of Active Galactic Nuclei (AGN)
obtained by combining X-ray and optical spectroscopic selections. Based on a
careful study of their Spectral Energy Distribution (SED), which has been
parametrized using a 2-component (AGN+galaxy) model fit, we derived
dust-corrected rest-frame magnitudes, colors, stellar masses and star formation
rates (SFRs). We find that AGN hosts span a large range of stellar masses and
SFRs. No color-bimodality is seen at any redshift in the AGN hosts, which are
found to be mainly massive, red galaxies. Once accounting for the color-mass
degeneracy in well defined mass-matched samples, we find a residual marginal
enhancement of AGN incidence in redder galaxies with lower specific star
formation rates, and we argue that this result might emerge because of our
ability to properly account for AGN light contamination and dust extinction.
Interestingly, we find that the probability for a galaxy to host a black hole
growing at any given "specific accretion rate" (i.e. the ratio of X-ray
luminosity to the host stellar mass) is almost independent of the host galaxy
mass, while it decreases as a power-law with Lx/M. By analyzing the
normalization of such probability distribution, we show how the incidence of
AGN increases with redshift as rapidly as (1+z)^4, in close resemblance with
the overall evolution of the specific star formation rate of the entire galaxy
population. Although AGN activity and star formation in galaxies do appear to
have a common triggering mechanism, at least in a statistical sense, within the
COSMOS sample we do not find strong evidence of any 'smoking gun' signaling
powerful AGN influence on the star-forming properties of their hosts galaxies.
Using the wide multi-band photometry available in the COSMOS field we explore
the host galaxy properties of a large sample of Active Galactic Nuclei (AGN)
obtained by combining X-ray and optical spectroscopic selections. Based on a
careful study of their Spectral Energy Distribution (SED), which has been
parametrized using a 2-component (AGN+galaxy) model fit, we derived
dust-corrected rest-frame magnitudes, colors, stellar masses and star formation
rates (SFRs). We find that AGN hosts span a large range of stellar masses and
SFRs. No color-bimodality is seen at any redshift in the AGN hosts, which are
found to be mainly massive, red galaxies. Once accounting for the color-mass
degeneracy in well defined mass-matched samples, we find a residual marginal
enhancement of AGN incidence in redder galaxies with lower specific star
formation rates, and we argue that this result might emerge because of our
ability to properly account for AGN light contamination and dust extinction.
Interestingly, we find that the probability for a galaxy to host a black hole
growing at any given "specific accretion rate" (i.e. the ratio of X-ray
luminosity to the host stellar mass) is almost independent of the host galaxy
mass, while it decreases as a power-law with Lx/M. By analyzing the
normalization of such probability distribution, we show how the incidence of
AGN increases with redshift as rapidly as (1+z)^4, in close resemblance with
the overall evolution of the specific star formation rate of the entire galaxy
population. Although AGN activity and star formation in galaxies do appear to
have a common triggering mechanism, at least in a statistical sense, within the
COSMOS sample we do not find strong evidence of any 'smoking gun' signaling
powerful AGN influence on the star-forming properties of their hosts galaxies.
Correlations in Nuclear Matter. (arXiv:1209.2270v1 [nucl-th])
Correlations in Nuclear Matter. (arXiv:1209.2270v1 [nucl-th]):
We analyze the nuclear matter correlation properties in terms of the pair
correlation function. To this aim we systematically compare the results for the
variational method in the Lowest Order Constrained Variational (LOCV)
approximation and for the Bruekner-Hartree-Fock (BHF) scheme. A formal link
between the Jastrow correlation factor of LOCV and the Defect Function (DF) of
BHF is established and it is shown under which conditions and approximations
the two approaches are equivalent. From the numerical comparison it turns out
that the two correlation functions are quite close, which indicates in
particular that the DF is approximately local and momentum independent. The
Equations of State (EOS) of Nuclear Matter in the two approaches are also
compared. It is found that once the three-body forces (TBF) are introduced the
two EOS are fairly close, while the agreement between the correlation functions
holds with or without TBF.
We analyze the nuclear matter correlation properties in terms of the pair
correlation function. To this aim we systematically compare the results for the
variational method in the Lowest Order Constrained Variational (LOCV)
approximation and for the Bruekner-Hartree-Fock (BHF) scheme. A formal link
between the Jastrow correlation factor of LOCV and the Defect Function (DF) of
BHF is established and it is shown under which conditions and approximations
the two approaches are equivalent. From the numerical comparison it turns out
that the two correlation functions are quite close, which indicates in
particular that the DF is approximately local and momentum independent. The
Equations of State (EOS) of Nuclear Matter in the two approaches are also
compared. It is found that once the three-body forces (TBF) are introduced the
two EOS are fairly close, while the agreement between the correlation functions
holds with or without TBF.
Black Holes in the Early Universe. (arXiv:1209.2243v1 [astro-ph.CO])
Black Holes in the Early Universe. (arXiv:1209.2243v1 [astro-ph.CO]):
The existence of massive black holes was postulated in the sixties, when the
first quasars were discovered. In the late nineties their reality was proven
beyond doubt, in the Milky way and a handful nearby galaxies. Since then,
enormous theoretical and observational efforts have been made to understand the
astrophysics of massive black holes. We have discovered that some of the most
massive black holes known, weighing billions of solar masses, powered luminous
quasars within the first billion years of the Universe. The first massive black
holes must therefore have formed around the time the first stars and galaxies
formed. Dynamical evidence also indicates that black holes with masses of
millions to billions of solar masses ordinarily dwell in the centers of today's
galaxies. Massive black holes populate galaxy centers today, and shone as
quasars in the past; the quiescent black holes that we detect now in nearby
bulges are the dormant remnants of this fiery past. In this review we report on
basic, but critical, questions regarding the cosmological significance of
massive black holes. What physical mechanisms lead to the formation of the
first massive black holes? How massive were the initial massive black hole
seeds? When and where did they form? How is the growth of black holes linked to
that of their host galaxy? Answers to most of these questions are work in
progress, in the spirit of these Reports on Progress in Physics.
The existence of massive black holes was postulated in the sixties, when the
first quasars were discovered. In the late nineties their reality was proven
beyond doubt, in the Milky way and a handful nearby galaxies. Since then,
enormous theoretical and observational efforts have been made to understand the
astrophysics of massive black holes. We have discovered that some of the most
massive black holes known, weighing billions of solar masses, powered luminous
quasars within the first billion years of the Universe. The first massive black
holes must therefore have formed around the time the first stars and galaxies
formed. Dynamical evidence also indicates that black holes with masses of
millions to billions of solar masses ordinarily dwell in the centers of today's
galaxies. Massive black holes populate galaxy centers today, and shone as
quasars in the past; the quiescent black holes that we detect now in nearby
bulges are the dormant remnants of this fiery past. In this review we report on
basic, but critical, questions regarding the cosmological significance of
massive black holes. What physical mechanisms lead to the formation of the
first massive black holes? How massive were the initial massive black hole
seeds? When and where did they form? How is the growth of black holes linked to
that of their host galaxy? Answers to most of these questions are work in
progress, in the spirit of these Reports on Progress in Physics.
X-ray Bright Active Galactic Nuclei in Massive Galaxy Clusters I: Number Counts and Spatial Distribution. (arXiv:1209.2132v1 [astro-ph.CO])
X-ray Bright Active Galactic Nuclei in Massive Galaxy Clusters I: Number Counts and Spatial Distribution. (arXiv:1209.2132v1 [astro-ph.CO]):
We present an analysis of the X-ray bright point source population in 43
massive clusters of galaxies observed with the Chandra X-ray Observatory. We
have constructed a catalog of 4210 rigorously selected X-ray point sources in
these fields, which span a survey area of 4.2 square degrees. This catalog
reveals a clear excess of sources when compared to deep blank-field surveys,
which amounts to roughly 1 additional source per cluster, likely Active
Galactic Nuclei (AGN) associated with the clusters. The excess sources are
concentrated within the virial radii of the clusters, with the largest excess
observed near the cluster centers. The average radial profile of the excess
X-ray sources of the cluster are well described by a power law (N(r) ~ r^\beta)
with an index of \beta ~ -0.5. An initial analysis using literature results on
the mean profile of member galaxies in massive X-ray selected clusters
indicates that the fraction of galaxies hosting X-ray AGN rises with increasing
clustercentric radius, being approximately 5 to 10 times higher near the virial
radius than in the central regions. This trend is qualitatively similar to that
observed for star formation in cluster member galaxies.
We present an analysis of the X-ray bright point source population in 43
massive clusters of galaxies observed with the Chandra X-ray Observatory. We
have constructed a catalog of 4210 rigorously selected X-ray point sources in
these fields, which span a survey area of 4.2 square degrees. This catalog
reveals a clear excess of sources when compared to deep blank-field surveys,
which amounts to roughly 1 additional source per cluster, likely Active
Galactic Nuclei (AGN) associated with the clusters. The excess sources are
concentrated within the virial radii of the clusters, with the largest excess
observed near the cluster centers. The average radial profile of the excess
X-ray sources of the cluster are well described by a power law (N(r) ~ r^\beta)
with an index of \beta ~ -0.5. An initial analysis using literature results on
the mean profile of member galaxies in massive X-ray selected clusters
indicates that the fraction of galaxies hosting X-ray AGN rises with increasing
clustercentric radius, being approximately 5 to 10 times higher near the virial
radius than in the central regions. This trend is qualitatively similar to that
observed for star formation in cluster member galaxies.
The 21cm forest in the diffuse IGM as seen by LOFAR. (arXiv:1209.2615v1 [astro-ph.CO])
The 21cm forest in the diffuse IGM as seen by LOFAR. (arXiv:1209.2615v1 [astro-ph.CO]):
We discuss the feasibility of the detection of the 21cm forest in the diffuse
IGM with the radio telescope LOFAR. The optical depth to the 21cm line has been
derived using simulations of reionization which include detailed radiative
transfer of ionizing photons. We find that the spectra from reionization models
with similar total comoving hydrogen ionizing emissivity but different
frequency distribution look remarkably similar. Thus, unless the reionization
histories are very different from each other (e.g. a predominance of UV vs.
x-ray heating) we do not expect to distinguish them by means of observations of
the 21cm forest. Because the presence of a strong x-ray background would make
the detection of 21cm line absorption impossible, the lack of absorption could
be used as a probe of the presence/intensity of the x-ray background and the
thermal history of the universe. Along a random line of sight LOFAR could
detect a global suppression of the spectrum from z>12, when the IGM is still
mostly neutral and cold, in contrast with the more well-defined, albeit broad,
absorption features visible at lower redshift. Sharp, strong absorption
features associated with rare, high density pockets of gas could be detected
also at z~7 along preferential lines of sight.
We discuss the feasibility of the detection of the 21cm forest in the diffuse
IGM with the radio telescope LOFAR. The optical depth to the 21cm line has been
derived using simulations of reionization which include detailed radiative
transfer of ionizing photons. We find that the spectra from reionization models
with similar total comoving hydrogen ionizing emissivity but different
frequency distribution look remarkably similar. Thus, unless the reionization
histories are very different from each other (e.g. a predominance of UV vs.
x-ray heating) we do not expect to distinguish them by means of observations of
the 21cm forest. Because the presence of a strong x-ray background would make
the detection of 21cm line absorption impossible, the lack of absorption could
be used as a probe of the presence/intensity of the x-ray background and the
thermal history of the universe. Along a random line of sight LOFAR could
detect a global suppression of the spectrum from z>12, when the IGM is still
mostly neutral and cold, in contrast with the more well-defined, albeit broad,
absorption features visible at lower redshift. Sharp, strong absorption
features associated with rare, high density pockets of gas could be detected
also at z~7 along preferential lines of sight.
Pure Neutron Matter Constraints and Nuclear Symmetry Energy. (arXiv:1209.2718v1 [nucl-th])
Pure Neutron Matter Constraints and Nuclear Symmetry Energy. (arXiv:1209.2718v1 [nucl-th]):
In this review, we will discuss the results of our recent work to study the
general optimization of the pure isovector parameters of the popular
relativistic mean-field (RMF) and Skyrme-Hartree-Fock (SHF) nuclear
energy-density functionals (EDFs), using constraints on the pure neutron matter
(PNM) equation of state (EoS) from recent {\sl ab initio} calculations. By
using RMF and SHF parameterizations that give equivalent predictions for
ground-state properties of doubly magic nuclei and properties of symmetric
nuclear matter (SNM) and PNM, we found that such optimization leads to broadly
consistent symmetry energy $J$ and its slope parameter $L$ at saturation
density within a tight range of $\sigma(J) < 2$ MeV and $\sigma(L) < 6$ MeV. We
demonstrate that a clear model dependence shows up (a) in the curvature
parameter of the symmetry energy $K_{\rm sym}$, (b) the symmetry energy at
supra-saturation densities, and (c) the radius of neutron stars.
In this review, we will discuss the results of our recent work to study the
general optimization of the pure isovector parameters of the popular
relativistic mean-field (RMF) and Skyrme-Hartree-Fock (SHF) nuclear
energy-density functionals (EDFs), using constraints on the pure neutron matter
(PNM) equation of state (EoS) from recent {\sl ab initio} calculations. By
using RMF and SHF parameterizations that give equivalent predictions for
ground-state properties of doubly magic nuclei and properties of symmetric
nuclear matter (SNM) and PNM, we found that such optimization leads to broadly
consistent symmetry energy $J$ and its slope parameter $L$ at saturation
density within a tight range of $\sigma(J) < 2$ MeV and $\sigma(L) < 6$ MeV. We
demonstrate that a clear model dependence shows up (a) in the curvature
parameter of the symmetry energy $K_{\rm sym}$, (b) the symmetry energy at
supra-saturation densities, and (c) the radius of neutron stars.
Monday, September 10, 2012
Discovery of an active supermassive black hole in the bulge-less galaxy NGC 4561. (arXiv:1209.1354v1 [astro-ph.CO])
Discovery of an active supermassive black hole in the bulge-less galaxy NGC 4561. (arXiv:1209.1354v1 [astro-ph.CO]):
We present XMM-Newton observations of the Chandra-detected nuclear X-ray
source in NGC 4561. The hard X-ray spectrum can be described by a model
composed of an absorbed power-law with Gamma= 2.5^{+0.4}_{-0.3}, and column
density N_H=1.9^{+0.1}_{-0.2} times 10^{22} atoms cm^{-2}. The absorption
corrected luminosity of the source is L(0.2 - 10.0 keV) = 2.5 times 10^{41}
ergs s^{-1}, with bolometric luminosity over 3 \times 10^{42} ergs s^{-1}.
Based on the spectrum and the luminosity, we identify the nuclear X-ray source
in NGC 4561 to be an AGN, with a black hole of mass M_BH > 20,000 solar masses.
The presence of a supermassive black hole at the center of this bulge-less
galaxy shows that black hole masses are not necessarily related to bulge
properties, contrary to the general belief. Observations such as these call
into question several theoretical models of BH--galaxy co-evolution that are
based on merger-driven BH growth; secular processes clearly play an important
role. Several emission lines are detected in the soft X-ray spectrum of the
source which can be well parametrized by an absorbed diffuse thermal plasma
with non-solar abundances of some heavy elements. Similar soft X-ray emission
is observed in spectra of Seyfert 2 galaxies and low luminosity AGNs,
suggesting an origin in the circumnuclear plasma.
We present XMM-Newton observations of the Chandra-detected nuclear X-ray
source in NGC 4561. The hard X-ray spectrum can be described by a model
composed of an absorbed power-law with Gamma= 2.5^{+0.4}_{-0.3}, and column
density N_H=1.9^{+0.1}_{-0.2} times 10^{22} atoms cm^{-2}. The absorption
corrected luminosity of the source is L(0.2 - 10.0 keV) = 2.5 times 10^{41}
ergs s^{-1}, with bolometric luminosity over 3 \times 10^{42} ergs s^{-1}.
Based on the spectrum and the luminosity, we identify the nuclear X-ray source
in NGC 4561 to be an AGN, with a black hole of mass M_BH > 20,000 solar masses.
The presence of a supermassive black hole at the center of this bulge-less
galaxy shows that black hole masses are not necessarily related to bulge
properties, contrary to the general belief. Observations such as these call
into question several theoretical models of BH--galaxy co-evolution that are
based on merger-driven BH growth; secular processes clearly play an important
role. Several emission lines are detected in the soft X-ray spectrum of the
source which can be well parametrized by an absorbed diffuse thermal plasma
with non-solar abundances of some heavy elements. Similar soft X-ray emission
is observed in spectra of Seyfert 2 galaxies and low luminosity AGNs,
suggesting an origin in the circumnuclear plasma.
Binary Paths to Type Ia Supernovae Explosions: The Highlights. (arXiv:1209.1201v1 [astro-ph.CO])
Binary Paths to Type Ia Supernovae Explosions: The Highlights. (arXiv:1209.1201v1 [astro-ph.CO]):
This Symposium was focused on the hunt for the progenitors of Supernovae of
Type Ia. Is there a main channel for the production of SNeIa? If so, are these
elusive progenitors Single Degenerate or Double Degenerate systems? Although
most participants seemed to favour the Single Degenerate channel, there was no
general agreement on the type of binary system at play. An observational puzzle
that was highlighted was the apparent paucity of Super-Soft Sources in our
Galaxy and also in external galaxies. The Single Degenerate channel (and as it
was pointed out, quite possibly also the Double Degenerate channel) requires
the binary system to pass through a phase of steady nuclear burning. However,
the observed number of Super-Soft sources falls short by a factor of up to 100
in explaining the estimated birth rates of SNeIa. Thus, are these Super-Soft
sources somehow hidden away and radiating at different wavelengths or are we
missing some important pieces of this puzzle that may lead to the elimination
of a certain class of progenitor? Another unanswered question concerns the
dependence of SNeIa luminosities on the age of their host galaxy. Several
hypotheses were put forward, but none was singled out as the most likely
explanation.
It is fair to say that at the end of the Symposium the definitive answer to
the vexed progenitor question remained is well and truly wide open.
This Symposium was focused on the hunt for the progenitors of Supernovae of
Type Ia. Is there a main channel for the production of SNeIa? If so, are these
elusive progenitors Single Degenerate or Double Degenerate systems? Although
most participants seemed to favour the Single Degenerate channel, there was no
general agreement on the type of binary system at play. An observational puzzle
that was highlighted was the apparent paucity of Super-Soft Sources in our
Galaxy and also in external galaxies. The Single Degenerate channel (and as it
was pointed out, quite possibly also the Double Degenerate channel) requires
the binary system to pass through a phase of steady nuclear burning. However,
the observed number of Super-Soft sources falls short by a factor of up to 100
in explaining the estimated birth rates of SNeIa. Thus, are these Super-Soft
sources somehow hidden away and radiating at different wavelengths or are we
missing some important pieces of this puzzle that may lead to the elimination
of a certain class of progenitor? Another unanswered question concerns the
dependence of SNeIa luminosities on the age of their host galaxy. Several
hypotheses were put forward, but none was singled out as the most likely
explanation.
It is fair to say that at the end of the Symposium the definitive answer to
the vexed progenitor question remained is well and truly wide open.
The first massive black holes. (arXiv:1209.1195v1 [astro-ph.CO])
The first massive black holes. (arXiv:1209.1195v1 [astro-ph.CO]):
I briefly outline recent theoretical developments on the formation of the
first massive black holes (MBHs) that may grow into the population of MBHs
powering quasars and inhabiting galactic centers today. I also touch upon
possible observational tests that may give insights on what the properties of
the first MBHs were.
Note: Brief but clear exposition on the relevant issues.
I briefly outline recent theoretical developments on the formation of the
first massive black holes (MBHs) that may grow into the population of MBHs
powering quasars and inhabiting galactic centers today. I also touch upon
possible observational tests that may give insights on what the properties of
the first MBHs were.
Note: Brief but clear exposition on the relevant issues.
Limits on the high redshift growth of massive black holes. (arXiv:1209.1095v1 [astro-ph.CO])
Limits on the high redshift growth of massive black holes. (arXiv:1209.1095v1 [astro-ph.CO]):
We place firm upper limits on the global accretion history of massive black
holes at z>5 from the recently measured unresolved fraction of the cosmic X-ray
background. The maximum allowed unresolved intensity observed at 1.5 keV
implies a maximum accreted-mass density onto massive black holes of rho_acc <
1.4E4 M_sun Mpc^{-3} for z>5. Considering the contribution of lower-z AGNs, the
value reduces to rho_acc < 0.66E4 M_sun Mpc^{-3}. The tension between the need
for the efficient and rapid accretion required by the observation of massive
black holes already in place at z>7 and the strict upper limit on the accreted
mass derived from the X-ray background may indicate that black holes are rare
in high redshift galaxies, or that accretion is efficient only for black holes
hosted by rare galaxies.
We place firm upper limits on the global accretion history of massive black
holes at z>5 from the recently measured unresolved fraction of the cosmic X-ray
background. The maximum allowed unresolved intensity observed at 1.5 keV
implies a maximum accreted-mass density onto massive black holes of rho_acc <
1.4E4 M_sun Mpc^{-3} for z>5. Considering the contribution of lower-z AGNs, the
value reduces to rho_acc < 0.66E4 M_sun Mpc^{-3}. The tension between the need
for the efficient and rapid accretion required by the observation of massive
black holes already in place at z>7 and the strict upper limit on the accreted
mass derived from the X-ray background may indicate that black holes are rare
in high redshift galaxies, or that accretion is efficient only for black holes
hosted by rare galaxies.
Precise Identifications of Submillimeter Galaxies: Measuring the History of Massive Star-Forming Galaxies to z>5. (arXiv:1209.1626v1 [astro-ph.CO])
Precise Identifications of Submillimeter Galaxies: Measuring the History of Massive Star-Forming Galaxies to z>5. (arXiv:1209.1626v1 [astro-ph.CO]):
We carried out extremely sensitive Submillimeter Array (SMA) 340 GHz (860
micron) continuum imaging of a complete sample of SCUBA 850 micron sources (>4
sigma) with fluxes >3 mJy in the GOODS-N. Using these data and new SCUBA-2
data, we find that 4 of the 16 SCUBA sources are spurious. A further 3 resolve
into multiple fainter SMA galaxies, suggesting that our understanding of the
most luminous high-redshift dusty galaxies may not be as reliable as we
thought. Ten of the 16 independent SMA sources have spectroscopic redshifts
(optical/infrared or CO) to z=5.18. Using a new, ultradeep 20 cm image obtained
with the Karl G. Jansky Very Large Array (rms of 2.5 microJy), we find that all
16 of the SMA sources are detected at >5 sigma. Using Herschel far-infrared
(FIR) data, we show that the 5 isolated SMA sources with Herschel detections
are well described by an Arp 220 spectral energy distribution template in the
FIR. They also closely obey the local FIR-radio correlation, a result that does
not suffer from a radio bias. We compute the contribution from the 16 SMA
sources to the universal star formation rate per comoving volume. With
individual star formation rates in the range 700-5000 solar masses per year,
they contribute ~30% of the extinction-corrected ultraviolet selected star
formation rate density from z=1 to at least z=5. Star formation histories
determined from extinction-corrected ultraviolet populations and from
submillimeter galaxy populations only partially overlap, due to the extreme
ultraviolet faintness of some submillimeter galaxies.
We carried out extremely sensitive Submillimeter Array (SMA) 340 GHz (860
micron) continuum imaging of a complete sample of SCUBA 850 micron sources (>4
sigma) with fluxes >3 mJy in the GOODS-N. Using these data and new SCUBA-2
data, we find that 4 of the 16 SCUBA sources are spurious. A further 3 resolve
into multiple fainter SMA galaxies, suggesting that our understanding of the
most luminous high-redshift dusty galaxies may not be as reliable as we
thought. Ten of the 16 independent SMA sources have spectroscopic redshifts
(optical/infrared or CO) to z=5.18. Using a new, ultradeep 20 cm image obtained
with the Karl G. Jansky Very Large Array (rms of 2.5 microJy), we find that all
16 of the SMA sources are detected at >5 sigma. Using Herschel far-infrared
(FIR) data, we show that the 5 isolated SMA sources with Herschel detections
are well described by an Arp 220 spectral energy distribution template in the
FIR. They also closely obey the local FIR-radio correlation, a result that does
not suffer from a radio bias. We compute the contribution from the 16 SMA
sources to the universal star formation rate per comoving volume. With
individual star formation rates in the range 700-5000 solar masses per year,
they contribute ~30% of the extinction-corrected ultraviolet selected star
formation rate density from z=1 to at least z=5. Star formation histories
determined from extinction-corrected ultraviolet populations and from
submillimeter galaxy populations only partially overlap, due to the extreme
ultraviolet faintness of some submillimeter galaxies.
AGN feedback and triggering of star formation in galaxies. (arXiv:1209.1480v1 [astro-ph.GA])
AGN feedback and triggering of star formation in galaxies. (arXiv:1209.1480v1 [astro-ph.GA]):
Feedback from the central black hole in active galactic nuclei (AGN) may be
responsible for establishing the observed MBH-sigma relation and limiting the
bulge stellar mass of the host galaxy. Here we explore the possibility of AGN
feedback triggering star formation in the host galaxy. We consider a shell of
dusty gas, driven outwards by radiation pressure, and analyse its
escape/trapping condition in the galactic halo for different underlying dark
matter potentials. In the isothermal potential, we obtain that the standard
condition setting the observed MBH-sigma relation is not sufficient to clear
gas out of the entire galaxy; whereas the same condition is formally sufficient
in the case of the Hernquist and Navarro-Frenk-White profiles. The squeezing
and compression of the inhomogeneous interstellar medium during the ejection
process can trigger star formation within the feedback-driven shell. We
estimate the resulting star formation rate and total additional stellar mass.
In this picture, new stars are formed at increasingly larger radii and
successively populate the outer regions of the host galaxy. This characteristic
pattern may be compared with the observed 'inside-out' growth of massive
galaxies.
Feedback from the central black hole in active galactic nuclei (AGN) may be
responsible for establishing the observed MBH-sigma relation and limiting the
bulge stellar mass of the host galaxy. Here we explore the possibility of AGN
feedback triggering star formation in the host galaxy. We consider a shell of
dusty gas, driven outwards by radiation pressure, and analyse its
escape/trapping condition in the galactic halo for different underlying dark
matter potentials. In the isothermal potential, we obtain that the standard
condition setting the observed MBH-sigma relation is not sufficient to clear
gas out of the entire galaxy; whereas the same condition is formally sufficient
in the case of the Hernquist and Navarro-Frenk-White profiles. The squeezing
and compression of the inhomogeneous interstellar medium during the ejection
process can trigger star formation within the feedback-driven shell. We
estimate the resulting star formation rate and total additional stellar mass.
In this picture, new stars are formed at increasingly larger radii and
successively populate the outer regions of the host galaxy. This characteristic
pattern may be compared with the observed 'inside-out' growth of massive
galaxies.
Tearing up the disc: how black holes accrete. (arXiv:1209.1393v1 [astro-ph.HE])
Tearing up the disc: how black holes accrete. (arXiv:1209.1393v1 [astro-ph.HE]):
We show that in realistic cases of accretion in active galactic nuclei or
stellar-mass X-ray binaries, the Lense-Thirring effect breaks the central
regions of tilted accretion discs around spinning black holes into a set of
distinct planes with only tenuous flows connecting them. If the original
misalignment of the outer disc to the spin axis of the hole is $45^{\circ}
\lesssim \theta \lesssim 135^{\circ}$, as in $\sim 70$% of randomly oriented
accretion events, the continued precession of these discs sets up partially
counter-rotating gas flows. This drives rapid infall as angular momentum is
cancelled and gas attempts to circularize at smaller radii. Disc breaking close
to the black hole leads to direct dynamical accretion, while breaking further
out can drive gas down to scales where it can accrete rapidly. For smaller tilt
angles breaking can still occur, and may lead to other observable phenomena
such as QPOs. For such effects not to appear, the black hole spin must in
practice be negligibly small, or be almost precisely aligned with the disc.
Qualitatively similar results hold for any accretion disc subject to a forced
differential precession, such as an external disc around a misaligned black
hole binary.
We show that in realistic cases of accretion in active galactic nuclei or
stellar-mass X-ray binaries, the Lense-Thirring effect breaks the central
regions of tilted accretion discs around spinning black holes into a set of
distinct planes with only tenuous flows connecting them. If the original
misalignment of the outer disc to the spin axis of the hole is $45^{\circ}
\lesssim \theta \lesssim 135^{\circ}$, as in $\sim 70$% of randomly oriented
accretion events, the continued precession of these discs sets up partially
counter-rotating gas flows. This drives rapid infall as angular momentum is
cancelled and gas attempts to circularize at smaller radii. Disc breaking close
to the black hole leads to direct dynamical accretion, while breaking further
out can drive gas down to scales where it can accrete rapidly. For smaller tilt
angles breaking can still occur, and may lead to other observable phenomena
such as QPOs. For such effects not to appear, the black hole spin must in
practice be negligibly small, or be almost precisely aligned with the disc.
Qualitatively similar results hold for any accretion disc subject to a forced
differential precession, such as an external disc around a misaligned black
hole binary.
The Density Profiles of Massive, Relaxed Galaxy Clusters: II. Separating Luminous and Dark Matter in Cluster Cores. (arXiv:1209.1392v1 [astro-ph.CO])
The Density Profiles of Massive, Relaxed Galaxy Clusters: II. Separating Luminous and Dark Matter in Cluster Cores. (arXiv:1209.1392v1 [astro-ph.CO]):
We present stellar and dark matter (DM) density profiles for a sample of 7
massive, relaxed galaxy clusters derived from strong and weak gravitational
lensing and resolved stellar kinematic observations within the
centrally-located brightest cluster galaxies (BCGs). In Paper I of the series,
we demonstrated that the total density profile derived from these data, which
span 3 decades in radius, is consistent with numerical DM-only simulations at
radii \gtrsim 5-10 kpc, despite the significant contribution of stellar
material in the core. Here we decompose the inner mass profiles of these
clusters into stellar and dark components. Parametrizing the DM density profile
as a power law rho_DM \sim r^{-\beta} on small scales, we find a mean slope
<\beta> = 0.50 +- 0.10 (random) +0.14-0.13 (systematic). Alternatively, cored
Navarro-Frenk-White (NFW) profiles with <log rcore/kpc> = 1.14 +- 0.13 (rand.)
+0.14-0.22 (sys.) provide an equally good description. These density profiles
are significantly shallower than canonical NFW models at radii \gtrsim 30 kpc,
comparable to the effective radii of the BCGs. The inner DM profile is
correlated with the distribution of stars in the BCG, demonstrating a close
connection between the inner halo and the assembly of stars in the central
galaxy. The stellar mass-to-light ratio inferred from lensing and stellar
dynamics is consistent with that inferred using stellar population synthesis
models if a Salpeter initial mass function is adopted. We compare these results
to theories describing the interaction between baryons and DM in cluster cores,
including prescriptions for adiabatic contraction, and discuss possible
signatures of alternative DM candidates.
We present stellar and dark matter (DM) density profiles for a sample of 7
massive, relaxed galaxy clusters derived from strong and weak gravitational
lensing and resolved stellar kinematic observations within the
centrally-located brightest cluster galaxies (BCGs). In Paper I of the series,
we demonstrated that the total density profile derived from these data, which
span 3 decades in radius, is consistent with numerical DM-only simulations at
radii \gtrsim 5-10 kpc, despite the significant contribution of stellar
material in the core. Here we decompose the inner mass profiles of these
clusters into stellar and dark components. Parametrizing the DM density profile
as a power law rho_DM \sim r^{-\beta} on small scales, we find a mean slope
<\beta> = 0.50 +- 0.10 (random) +0.14-0.13 (systematic). Alternatively, cored
Navarro-Frenk-White (NFW) profiles with <log rcore/kpc> = 1.14 +- 0.13 (rand.)
+0.14-0.22 (sys.) provide an equally good description. These density profiles
are significantly shallower than canonical NFW models at radii \gtrsim 30 kpc,
comparable to the effective radii of the BCGs. The inner DM profile is
correlated with the distribution of stars in the BCG, demonstrating a close
connection between the inner halo and the assembly of stars in the central
galaxy. The stellar mass-to-light ratio inferred from lensing and stellar
dynamics is consistent with that inferred using stellar population synthesis
models if a Salpeter initial mass function is adopted. We compare these results
to theories describing the interaction between baryons and DM in cluster cores,
including prescriptions for adiabatic contraction, and discuss possible
signatures of alternative DM candidates.
The Density Profiles of Massive, Relaxed Galaxy Clusters: I. The Total Density Over 3 Decades in Radius. (arXiv:1209.1391v1 [astro-ph.CO])
The Density Profiles of Massive, Relaxed Galaxy Clusters: I. The Total Density Over 3 Decades in Radius. (arXiv:1209.1391v1 [astro-ph.CO]):
Clusters of galaxies are excellent locations to probe the distribution of
baryons and dark matter over a wide range of scales. We study a sample of 7
massive, relaxed galaxy clusters with centrally-located brightest cluster
galaxies (BCGs) at z=0.2-0.3. Using the observational tools of strong and weak
gravitational lensing, combined with resolved stellar kinematics within the
BCG, we measure the total radial density profile, comprising both dark and
baryonic matter, over scales of \sim3-3000 kpc. Lensing-derived mass profiles
typically agree with independent X-ray estimates within \sim15%, suggesting
that departures from hydrostatic equilibrium are small and that the clusters in
our sample (except A383) are not strongly elongated along the line of sight.
The inner logarithmic slope gamma_tot of the total density profile measured
over r/r200=0.003-0.03, where rho_tot \sim r^(-gamma_tot), is found to be
nearly universal, with a mean <gamma_tot> = 1.16 +- 0.05 (random) +0.05-0.07
(systematic) and an intrinsic scatter of < 0.13 (68% confidence). This is
further supported by the very homogeneous shape of the observed velocity
dispersion profiles, obtained via Keck spectroscopy, which are mutually
consistent after a simple scaling. Remarkably, this slope agrees closely with
numerical simulations that contain only dark matter, despite the significant
contribution of stellar mass on the scales we probe. The Navarro-Frenk-White
profile characteristic of collisionless cold dark matter is a better
description of the total mass density at radii \gtrsim 5-10 kpc than that of
dark matter alone. Hydrodynamical simulations that include baryons, cooling,
and feedback currently provide a poorer match. We discuss the significance of
our findings for understanding the assembly of BCGs and cluster cores,
particularly the influence of baryons on the inner dark matter halo. [abridged]
Clusters of galaxies are excellent locations to probe the distribution of
baryons and dark matter over a wide range of scales. We study a sample of 7
massive, relaxed galaxy clusters with centrally-located brightest cluster
galaxies (BCGs) at z=0.2-0.3. Using the observational tools of strong and weak
gravitational lensing, combined with resolved stellar kinematics within the
BCG, we measure the total radial density profile, comprising both dark and
baryonic matter, over scales of \sim3-3000 kpc. Lensing-derived mass profiles
typically agree with independent X-ray estimates within \sim15%, suggesting
that departures from hydrostatic equilibrium are small and that the clusters in
our sample (except A383) are not strongly elongated along the line of sight.
The inner logarithmic slope gamma_tot of the total density profile measured
over r/r200=0.003-0.03, where rho_tot \sim r^(-gamma_tot), is found to be
nearly universal, with a mean <gamma_tot> = 1.16 +- 0.05 (random) +0.05-0.07
(systematic) and an intrinsic scatter of < 0.13 (68% confidence). This is
further supported by the very homogeneous shape of the observed velocity
dispersion profiles, obtained via Keck spectroscopy, which are mutually
consistent after a simple scaling. Remarkably, this slope agrees closely with
numerical simulations that contain only dark matter, despite the significant
contribution of stellar mass on the scales we probe. The Navarro-Frenk-White
profile characteristic of collisionless cold dark matter is a better
description of the total mass density at radii \gtrsim 5-10 kpc than that of
dark matter alone. Hydrodynamical simulations that include baryons, cooling,
and feedback currently provide a poorer match. We discuss the significance of
our findings for understanding the assembly of BCGs and cluster cores,
particularly the influence of baryons on the inner dark matter halo. [abridged]
Thursday, September 6, 2012
Black hole bombs and photon mass bounds. (arXiv:1209.0465v1 [gr-qc] CROSS LISTED)
Black hole bombs and photon mass bounds. (arXiv:1209.0465v1 [gr-qc] CROSS LISTED):
Generic extensions of the standard model predict the existence of ultralight
bosonic degrees of freedom. Several ongoing experiments are aimed at detecting
these particles or constraining their mass range. Here we show that massive
vector fields around rotating black holes can give rise to a strong
superradiant instability which extracts angular momentum from the hole. The
observation of supermassive spinning black holes imposes limits on this
mechanism. We show that current supermassive black hole spin estimates provide
the tightest upper limits on the mass of the photon (mv<4x10^{-20} eV according
to our most conservative estimate), and that spin measurements for the largest
known supermassive black holes could further lower this bound to mv<10^{-22}
eV. Our analysis relies on a novel framework to study perturbations of rotating
Kerr black holes in the slow-rotation regime, that we developed up to second
order in rotation, and that can be extended to other spacetime metrics and
other theories.
Generic extensions of the standard model predict the existence of ultralight
bosonic degrees of freedom. Several ongoing experiments are aimed at detecting
these particles or constraining their mass range. Here we show that massive
vector fields around rotating black holes can give rise to a strong
superradiant instability which extracts angular momentum from the hole. The
observation of supermassive spinning black holes imposes limits on this
mechanism. We show that current supermassive black hole spin estimates provide
the tightest upper limits on the mass of the photon (mv<4x10^{-20} eV according
to our most conservative estimate), and that spin measurements for the largest
known supermassive black holes could further lower this bound to mv<10^{-22}
eV. Our analysis relies on a novel framework to study perturbations of rotating
Kerr black holes in the slow-rotation regime, that we developed up to second
order in rotation, and that can be extended to other spacetime metrics and
other theories.
White Dwarf/M Dwarf Binaries as Single Degenerate Progenitors of Type Ia Supernovae. (arXiv:1209.1021v1 [astro-ph.SR])
White Dwarf/M Dwarf Binaries as Single Degenerate Progenitors of Type Ia Supernovae. (arXiv:1209.1021v1 [astro-ph.SR]):
Limits on the companions of white dwarfs in the single degenerate scenario
for the origin of Type Ia supernovae (SNIa) have gotten increasingly tight. The
only type of non-degenerate stars that survive the limits on the companions of
SNIa in SNR 0509-67.5 and SN1572 are M dwarfs. M dwarfs have special properties
that have not been considered in most work on the progenitors of SNIa: they
have small but finite magnetic fields, and they flare frequently. These
properties are explored in the context of SNIa progenitors. White dwarf/M dwarf
pairs may be sufficiently plentiful to provide an adequate rate of explosions.
Even modest magnetic fields on the white dwarf and M dwarf will yield adequate
torques to lock the two stars together, resulting in a slowly rotating white
dwarf, with the magnetic poles pointing at one another in the orbital plane.
The mass loss will be channeled by a "magnetic bottle" connecting the two
stars, landing on a concentrated polar area on the white dwarf. This enhances
the effective rate of accretion compared to spherical accretion. Luminosity
from accretion and hydrogen burning on the surface of the white dwarf may
induce self-excited mass transfer. The combined effects of self-excited mass
loss, polar accretion, and magnetic inhibition of mixing of accretion layers
give possible means to beat the "nova limit" and grow the white dwarf to the
Chandrasekhar mass even at rather moderate mass accretion rates.
Limits on the companions of white dwarfs in the single degenerate scenario
for the origin of Type Ia supernovae (SNIa) have gotten increasingly tight. The
only type of non-degenerate stars that survive the limits on the companions of
SNIa in SNR 0509-67.5 and SN1572 are M dwarfs. M dwarfs have special properties
that have not been considered in most work on the progenitors of SNIa: they
have small but finite magnetic fields, and they flare frequently. These
properties are explored in the context of SNIa progenitors. White dwarf/M dwarf
pairs may be sufficiently plentiful to provide an adequate rate of explosions.
Even modest magnetic fields on the white dwarf and M dwarf will yield adequate
torques to lock the two stars together, resulting in a slowly rotating white
dwarf, with the magnetic poles pointing at one another in the orbital plane.
The mass loss will be channeled by a "magnetic bottle" connecting the two
stars, landing on a concentrated polar area on the white dwarf. This enhances
the effective rate of accretion compared to spherical accretion. Luminosity
from accretion and hydrogen burning on the surface of the white dwarf may
induce self-excited mass transfer. The combined effects of self-excited mass
loss, polar accretion, and magnetic inhibition of mixing of accretion layers
give possible means to beat the "nova limit" and grow the white dwarf to the
Chandrasekhar mass even at rather moderate mass accretion rates.
Plunging fireworks: Why do infalling galaxies light up on the outskirts of clusters?. (arXiv:1209.0972v1 [astro-ph.CO])
Plunging fireworks: Why do infalling galaxies light up on the outskirts of clusters?. (arXiv:1209.0972v1 [astro-ph.CO]):
(abridged)Integrated star formation rate (SFR) and specific star formation
rate (sSFR), derived from the spectroscopic data obtained by SDSS DR4 are used
to show that the star formation activity in galaxies (M_r<=-20.5) found on the
outskirts (1-2r_{200}) of some nearby clusters (0.02<=z<=0.15) is enhanced. By
comparing the mean SFR of galaxies in a sample of clusters with at least one
starburst galaxy (log sSFR>=-10/yr & SFR>10 M_sun/yr) to a sample of clusters
without such galaxies (`comparison' clusters), we find that despite the
expected decline in the mean SFR of galaxies toward the cluster core, the SFR
profile of the two samples is different, such that the galaxies in the
`comparison' clusters show a lower mean SFR at all radius (<=3r_{200}) from the
cluster centre. Such an increase in the SFR of galaxies is more likely to be
seen in dynamically unrelaxed (sigma_v>~500 km/s) clusters. It is also evident
that these unrelaxed clusters are currently being assembled via galaxies
falling in through straight filaments, resulting in high velocity dispersions.
On the other hand, `comparison' clusters are more likely to be fed by
relatively low density filaments. We find that the starburst galaxies on the
periphery of clusters are in an environment of higher local density than other
cluster galaxies at similar radial distances from the cluster centre. We
conclude that a relatively high galaxy density in the infalling regions of
clusters promotes interactions amongst galaxies, leading to momentary bursts of
star formation. Such interactions play a crucial role in exhausting the fuel
for star formation in a galaxy, before it is expelled due to the environmental
processes that are operational in the dense interiors of the cluster.
(abridged)Integrated star formation rate (SFR) and specific star formation
rate (sSFR), derived from the spectroscopic data obtained by SDSS DR4 are used
to show that the star formation activity in galaxies (M_r<=-20.5) found on the
outskirts (1-2r_{200}) of some nearby clusters (0.02<=z<=0.15) is enhanced. By
comparing the mean SFR of galaxies in a sample of clusters with at least one
starburst galaxy (log sSFR>=-10/yr & SFR>10 M_sun/yr) to a sample of clusters
without such galaxies (`comparison' clusters), we find that despite the
expected decline in the mean SFR of galaxies toward the cluster core, the SFR
profile of the two samples is different, such that the galaxies in the
`comparison' clusters show a lower mean SFR at all radius (<=3r_{200}) from the
cluster centre. Such an increase in the SFR of galaxies is more likely to be
seen in dynamically unrelaxed (sigma_v>~500 km/s) clusters. It is also evident
that these unrelaxed clusters are currently being assembled via galaxies
falling in through straight filaments, resulting in high velocity dispersions.
On the other hand, `comparison' clusters are more likely to be fed by
relatively low density filaments. We find that the starburst galaxies on the
periphery of clusters are in an environment of higher local density than other
cluster galaxies at similar radial distances from the cluster centre. We
conclude that a relatively high galaxy density in the infalling regions of
clusters promotes interactions amongst galaxies, leading to momentary bursts of
star formation. Such interactions play a crucial role in exhausting the fuel
for star formation in a galaxy, before it is expelled due to the environmental
processes that are operational in the dense interiors of the cluster.
The XMM deep survey in the CDF-S II. a 9-20 keV selection of heavily obscured active galaxies at z>1.7. (arXiv:1209.0916v1 [astro-ph.CO])
The XMM deep survey in the CDF-S II. a 9-20 keV selection of heavily obscured active galaxies at z>1.7. (arXiv:1209.0916v1 [astro-ph.CO]):
We present results on a search of heavily obscured active galaxies z>1.7
using the rest-frame 9-20 keV excess for X-ray sources detected in the deep
XMM-CDFS survey. Out of 176 sources selected with the conservative detection
criteria (>8 sigma) in the first source catalogue of Ranalli et al., 46 objects
lie in the redshift range of interest with the median redshift z~2.5. Their
typical rest-frame 10-20 keV luminosity is 1e+44 erg/s, as observed. Among
optically faint objects that lack spectroscopic redshift, four were found to be
strongly absorbed X-ray sources, and the enhanced Fe K emission or absorption
features in their X-ray spectra were used to obtain X-ray spectroscopic
redshifts. Using the X-ray colour-colour diagram based on the rest-frame 3-5
keV, 5-9 keV, and 9-20 keV bands, seven objects were selected for their 9-20
keV excess and were found to be strongly absorbed X-ray sources with column
density of nH > 0.6e+24 cm-2, including two possible Compton thick sources.
While they are emitting at quasar luminosity, ~3/4 of the sample objects are
found to be absorbed by nH > 1e+22 cm-2. A comparison with local AGN at the
matched luminosity suggests an increasing trend of the absorbed source fraction
for high-luminosity AGN towards high redshifts.
We present results on a search of heavily obscured active galaxies z>1.7
using the rest-frame 9-20 keV excess for X-ray sources detected in the deep
XMM-CDFS survey. Out of 176 sources selected with the conservative detection
criteria (>8 sigma) in the first source catalogue of Ranalli et al., 46 objects
lie in the redshift range of interest with the median redshift z~2.5. Their
typical rest-frame 10-20 keV luminosity is 1e+44 erg/s, as observed. Among
optically faint objects that lack spectroscopic redshift, four were found to be
strongly absorbed X-ray sources, and the enhanced Fe K emission or absorption
features in their X-ray spectra were used to obtain X-ray spectroscopic
redshifts. Using the X-ray colour-colour diagram based on the rest-frame 3-5
keV, 5-9 keV, and 9-20 keV bands, seven objects were selected for their 9-20
keV excess and were found to be strongly absorbed X-ray sources with column
density of nH > 0.6e+24 cm-2, including two possible Compton thick sources.
While they are emitting at quasar luminosity, ~3/4 of the sample objects are
found to be absorbed by nH > 1e+22 cm-2. A comparison with local AGN at the
matched luminosity suggests an increasing trend of the absorbed source fraction
for high-luminosity AGN towards high redshifts.
Pop III GRBs: an estimative of the event rate for future surveys. (arXiv:1209.0823v1 [astro-ph.CO])
Pop III GRBs: an estimative of the event rate for future surveys. (arXiv:1209.0823v1 [astro-ph.CO]):
We discuss the theoretical event rate of gamma-ray bursts (GRBs) from the
collapse of massive primordial stars. We construct a theoretical model to
calculate the rate and detectability of these GRBs taking into account all
important feedback and recent results from numerical simulations of pristine
gas. We expect to observe a maximum of N $\lesssim$ 0.2 GRBs per year
integrated over at z > 6 with \textit{Swift} and N $\lesssim$ 10 GRBs per year
integrated over at z > 6 with EXIST.
We discuss the theoretical event rate of gamma-ray bursts (GRBs) from the
collapse of massive primordial stars. We construct a theoretical model to
calculate the rate and detectability of these GRBs taking into account all
important feedback and recent results from numerical simulations of pristine
gas. We expect to observe a maximum of N $\lesssim$ 0.2 GRBs per year
integrated over at z > 6 with \textit{Swift} and N $\lesssim$ 10 GRBs per year
integrated over at z > 6 with EXIST.
General Relativistic Simulations of Accretion Induced Collapse of Neutron Stars to Black Holes. (arXiv:1209.0783v1 [astro-ph.HE])
General Relativistic Simulations of Accretion Induced Collapse of Neutron Stars to Black Holes. (arXiv:1209.0783v1 [astro-ph.HE]):
Neutron stars (NSs) in the astrophysical Universe are often surrounded by
accretion disks. Accretion of matter onto a NS may increase its mass above the
maximum value allowed by its equation of state, inducing its collapse to a
black hole (BH). Here we study this process for the first time, in 3D, and in
full general relativity. By considering three initial NS configurations, each
with and without a surrounding disk (of mass ~7% M_{NS}), we investigate the
effect of the accretion disk on the dynamics of the collapse and its imprint on
both the gravitational wave (GW) and electromagnetic (EM) signals that can be
emitted by these sources. We show in particular that, even if the GW signal is
similar for the accretion induced collapse (AIC) and the collapse of a NS in
vacuum (and detectable only for Galactic sources), the EM counterpart could
allow to discriminate between these two types of events. In fact, our
simulations show that, while the collapse of a NS in vacuum leaves no
appreciable baryonic matter outside the event horizon, an AIC is followed by a
phase of rapid accretion of the surviving disk onto the newly formed BH. The
post-collapse accretion rates, on the order of ~10^{-2} M_{sun} s^{-1}, make
these events tantalizing candidates as engines of short Gamma-Ray Bursts.
Neutron stars (NSs) in the astrophysical Universe are often surrounded by
accretion disks. Accretion of matter onto a NS may increase its mass above the
maximum value allowed by its equation of state, inducing its collapse to a
black hole (BH). Here we study this process for the first time, in 3D, and in
full general relativity. By considering three initial NS configurations, each
with and without a surrounding disk (of mass ~7% M_{NS}), we investigate the
effect of the accretion disk on the dynamics of the collapse and its imprint on
both the gravitational wave (GW) and electromagnetic (EM) signals that can be
emitted by these sources. We show in particular that, even if the GW signal is
similar for the accretion induced collapse (AIC) and the collapse of a NS in
vacuum (and detectable only for Galactic sources), the EM counterpart could
allow to discriminate between these two types of events. In fact, our
simulations show that, while the collapse of a NS in vacuum leaves no
appreciable baryonic matter outside the event horizon, an AIC is followed by a
phase of rapid accretion of the surviving disk onto the newly formed BH. The
post-collapse accretion rates, on the order of ~10^{-2} M_{sun} s^{-1}, make
these events tantalizing candidates as engines of short Gamma-Ray Bursts.
Galaxy Gas Fractions at High-Redshift: The Tension between Observations and Cosmological Simulations. (arXiv:1209.0771v1 [astro-ph.CO])
Galaxy Gas Fractions at High-Redshift: The Tension between Observations and Cosmological Simulations. (arXiv:1209.0771v1 [astro-ph.CO]):
CO measurements of z~1-4 galaxies have found that their baryonic gas
fractions are significantly higher than galaxies at z=0, with values ranging
from 20-80 %. Here, we suggest that the gas fractions inferred from
observations of star-forming galaxies at high-z are overestimated, owing to the
adoption of locally-calibrated CO-H2 conversion factors (Xco). Evidence from
both observations and numerical models suggest that Xco varies smoothly with
the physical properties of galaxies, and that Xco can be parameterised simply
as a function of both gas phase metallicity and observed CO surface brightness.
When applying this functional form, we find fgas ~10-40 % in galaxies with
M*=10^10-10^12 Msun at high-z. Moreover, the scatter in the observed fgas-M*
relation is lowered by a factor of two. The lower inferred gas fractions arise
physically because the interstellar media of high-z galaxies have higher
velocity dispersions and gas temperatures than their local counterparts, which
results in an Xco that is lower than the z=0 value for both quiescent discs and
starbursts. We further compare these gas fractions to those predicted by
cosmological galaxy formation models. We show that while the canonically
inferred gas fractions from observations are a factor of 2-3 larger at a given
stellar mass than predicted by models, our rederived Xco values for z=1-4
galaxies results in revised gas fractions that agree significantly better with
the simulations.
CO measurements of z~1-4 galaxies have found that their baryonic gas
fractions are significantly higher than galaxies at z=0, with values ranging
from 20-80 %. Here, we suggest that the gas fractions inferred from
observations of star-forming galaxies at high-z are overestimated, owing to the
adoption of locally-calibrated CO-H2 conversion factors (Xco). Evidence from
both observations and numerical models suggest that Xco varies smoothly with
the physical properties of galaxies, and that Xco can be parameterised simply
as a function of both gas phase metallicity and observed CO surface brightness.
When applying this functional form, we find fgas ~10-40 % in galaxies with
M*=10^10-10^12 Msun at high-z. Moreover, the scatter in the observed fgas-M*
relation is lowered by a factor of two. The lower inferred gas fractions arise
physically because the interstellar media of high-z galaxies have higher
velocity dispersions and gas temperatures than their local counterparts, which
results in an Xco that is lower than the z=0 value for both quiescent discs and
starbursts. We further compare these gas fractions to those predicted by
cosmological galaxy formation models. We show that while the canonically
inferred gas fractions from observations are a factor of 2-3 larger at a given
stellar mass than predicted by models, our rederived Xco values for z=1-4
galaxies results in revised gas fractions that agree significantly better with
the simulations.
Narrow Atomic Features from Rapidly Spinning Neutron Stars. (arXiv:1209.0768v1 [astro-ph.HE])
Narrow Atomic Features from Rapidly Spinning Neutron Stars. (arXiv:1209.0768v1 [astro-ph.HE]):
Neutron stars spinning at moderate rates (~300-600Hz) become oblate in shape
and acquire a nonzero quadrupole moment. In this paper, we calculate profiles
of atomic features from such neutron stars using a ray-tracing algorithm in the
Hartle-Thorne approximation. We show that line profiles acquire cores that are
much narrower than the widths expected from pure Doppler effects for a large
range of observer inclinations. As a result, the effects of both the oblateness
and the quadrupole moments of neutron stars need to be taken into account when
aiming to measure neutron star radii from rotationally broadened lines.
Moreover, the presence of these narrow cores substantially increases the
likelihood of detecting atomic lines from rapidly spinning neutron stars.
Neutron stars spinning at moderate rates (~300-600Hz) become oblate in shape
and acquire a nonzero quadrupole moment. In this paper, we calculate profiles
of atomic features from such neutron stars using a ray-tracing algorithm in the
Hartle-Thorne approximation. We show that line profiles acquire cores that are
much narrower than the widths expected from pure Doppler effects for a large
range of observer inclinations. As a result, the effects of both the oblateness
and the quadrupole moments of neutron stars need to be taken into account when
aiming to measure neutron star radii from rotationally broadened lines.
Moreover, the presence of these narrow cores substantially increases the
likelihood of detecting atomic lines from rapidly spinning neutron stars.
Wednesday, September 5, 2012
The X-ray variability history of Markarian 3. (arXiv:1209.0706v1 [astro-ph.CO])
The X-ray variability history of Markarian 3. (arXiv:1209.0706v1 [astro-ph.CO]):
We aim at constraining the geometry of the reprocessing matter in the nearby
prototypical Seyfert 2 Galaxy Markarian 3 by studying the time evolution of
spectral components associated to the primary AGN emission and to its
Compton-scattering. We have analyzed archival spectroscopic observations of
Markarian 3 taken over the last 12 years with the XMM-Newton, Suzaku and Swift
observatories, as well as data taken during a monitoring campaign activated by
us in 2012. The timescale of the Compton-reflection component variability
(originally discovered by ASCA in the mid-'90s) is ~64 days. This upper limit
improves by more than a factor of 15 previous estimates of the
Compton-reflection variability timescale for this source. When the light curve
of the Compton-reflection continuum in the 4-5 keV band is correlated with the
15-150 keV Swift/BAT curve a delay ~1200 days is found. The cross-correlation
results are dependent on the model used to fit the spectra, although the
detection of the Compton-reflection component variability is independent of the
range of models employed to fit the data. Reanalysis of an archival Chandra
image of Markarian 3 indicates that the Compton-reflection and the Fe K-alpha
emitting regions are extended to the North up to ~300 pc. The combination of
these findings suggests that the optically-thick reprocessor in Markarian 3 is
clumpy. There is mounting experimental evidence for the structure of the
optically-thick gas and dust in the nuclear environment of nearby heavily
obscured AGN to be extended and complex. We discuss possible modifications to
the standard unification scenarios encompassing this complexity. Markarian 3,
exhibiting X-ray absorption and reprocessing on widely different spatial
scales, is an ideal laboratory to test these models (abridged).
We aim at constraining the geometry of the reprocessing matter in the nearby
prototypical Seyfert 2 Galaxy Markarian 3 by studying the time evolution of
spectral components associated to the primary AGN emission and to its
Compton-scattering. We have analyzed archival spectroscopic observations of
Markarian 3 taken over the last 12 years with the XMM-Newton, Suzaku and Swift
observatories, as well as data taken during a monitoring campaign activated by
us in 2012. The timescale of the Compton-reflection component variability
(originally discovered by ASCA in the mid-'90s) is ~64 days. This upper limit
improves by more than a factor of 15 previous estimates of the
Compton-reflection variability timescale for this source. When the light curve
of the Compton-reflection continuum in the 4-5 keV band is correlated with the
15-150 keV Swift/BAT curve a delay ~1200 days is found. The cross-correlation
results are dependent on the model used to fit the spectra, although the
detection of the Compton-reflection component variability is independent of the
range of models employed to fit the data. Reanalysis of an archival Chandra
image of Markarian 3 indicates that the Compton-reflection and the Fe K-alpha
emitting regions are extended to the North up to ~300 pc. The combination of
these findings suggests that the optically-thick reprocessor in Markarian 3 is
clumpy. There is mounting experimental evidence for the structure of the
optically-thick gas and dust in the nuclear environment of nearby heavily
obscured AGN to be extended and complex. We discuss possible modifications to
the standard unification scenarios encompassing this complexity. Markarian 3,
exhibiting X-ray absorption and reprocessing on widely different spatial
scales, is an ideal laboratory to test these models (abridged).
FUV and X-ray irradiated protoplanetary disks: a grid of models II - Gas diagnostic line emission. (arXiv:1209.0591v1 [astro-ph.SR])
FUV and X-ray irradiated protoplanetary disks: a grid of models II - Gas diagnostic line emission. (arXiv:1209.0591v1 [astro-ph.SR]):
Most of the mass in protoplanetary disks is in the form of gas. The study of
the gas and its diagnostics is of fundamental importance in order to achieve a
detailed description of the thermal and chemical structure of the disk. The
radiation from the central star (from optical to X-ray wavelengths) and viscous
accretion are the main source of energy and dominates the disk physics and
chemistry in its early stages. This is the environment in which the first
phases of planet formation will proceed. We investigate how stellar and disk
parameters impact the fine-structure cooling lines [NeII], [ArII], [OI], [CII]
and H2O rotational lines in the disk. These lines are potentially powerful
diagnostics of the disk structure and their modelling permits a thorough
interpretation of the observations carried out with instrumental facilities
such as Spitzer and Herschel. Following Aresu et al. (2011), we computed a grid
of 240 disk models, in which the X-ray luminosity, UV-excess luminosity,
minimum dust grain size, dust size distribution power law and surface density
distribution power law, are systematically varied. We solve self-consistently
for the disk vertical hydrostatic structure in every model and apply detailed
line radiative transfer to calculate line fluxes and profiles for a series of
well known mid- and far-infrared cooling lines. The [OI] 63 micron line flux
increases with increasing FUV luminosity when Lx < 1e30 erg/s, and with
increasing X-ray luminosity when LX > 1e30 erg/s. [CII] 157 micron is mainly
driven by FUV luminosity via C+ production, X-rays affect the line flux to a
lesser extent. [NeII] 12.8 micron correlates with X-rays; the line profile
emitted from the disk atmosphere shows a double-peaked component, caused by
emission in the static disk atmosphere, next to a high velocity double-peaked
component, caused by emission in the very inner rim. (abridged)
Most of the mass in protoplanetary disks is in the form of gas. The study of
the gas and its diagnostics is of fundamental importance in order to achieve a
detailed description of the thermal and chemical structure of the disk. The
radiation from the central star (from optical to X-ray wavelengths) and viscous
accretion are the main source of energy and dominates the disk physics and
chemistry in its early stages. This is the environment in which the first
phases of planet formation will proceed. We investigate how stellar and disk
parameters impact the fine-structure cooling lines [NeII], [ArII], [OI], [CII]
and H2O rotational lines in the disk. These lines are potentially powerful
diagnostics of the disk structure and their modelling permits a thorough
interpretation of the observations carried out with instrumental facilities
such as Spitzer and Herschel. Following Aresu et al. (2011), we computed a grid
of 240 disk models, in which the X-ray luminosity, UV-excess luminosity,
minimum dust grain size, dust size distribution power law and surface density
distribution power law, are systematically varied. We solve self-consistently
for the disk vertical hydrostatic structure in every model and apply detailed
line radiative transfer to calculate line fluxes and profiles for a series of
well known mid- and far-infrared cooling lines. The [OI] 63 micron line flux
increases with increasing FUV luminosity when Lx < 1e30 erg/s, and with
increasing X-ray luminosity when LX > 1e30 erg/s. [CII] 157 micron is mainly
driven by FUV luminosity via C+ production, X-rays affect the line flux to a
lesser extent. [NeII] 12.8 micron correlates with X-rays; the line profile
emitted from the disk atmosphere shows a double-peaked component, caused by
emission in the static disk atmosphere, next to a high velocity double-peaked
component, caused by emission in the very inner rim. (abridged)
The enrichment history of the intracluster medium: a Bayesian approach. (arXiv:1209.0565v1 [astro-ph.CO])
The enrichment history of the intracluster medium: a Bayesian approach. (arXiv:1209.0565v1 [astro-ph.CO]):
This work measures the evolution of the iron content in galaxy clusters by a
rigorous analysis of the data of 130 clusters at 0.1<z<1.3. This task is made
difficult by a) the low signal-to-noise ratio of abundance measurements and the
upper limits, b) possible selection effects, c) boundaries in the parameter
space, d) non-Gaussian errors, e) the intrinsic variety of the objects studied,
and f) abundance systematics. We introduce a Bayesian model to address all
these issues at the same time, thus allowing cross-talk (covariance). On
simulated data, the Bayesian fit recovers the input enrichment history, unlike
in standard analysis. After accounting for a possible dependence on X-ray
temperature, for metal abundance systematics, and for the intrinsic variety of
studied objects, we found that the present-day metal content is not reached
either at high or at low redshifts, but gradually over time: iron abundance
increases by a factor 1.5 in the 7 Gyr sampled by the data. Therefore, feedback
in metal abundance does not end at high redshift. Evolution is established with
a moderate amount of evidence, 19 to 1 odds against faster or slower metal
enrichment histories. We quantify, for the first time, the intrinsic spread in
metal abundance, 18+/-3 %, after correcting for the effect of evolution, X-ray
temperature, and metal abundance systematics. Finally, we also present an
analytic approximation of the X-ray temperature and metal abundance likelihood
functions, which are useful for other regression fitting involving these
parameters. The data for the 130 clusters and code used for the stochastic
computation are provided with the paper.
This work measures the evolution of the iron content in galaxy clusters by a
rigorous analysis of the data of 130 clusters at 0.1<z<1.3. This task is made
difficult by a) the low signal-to-noise ratio of abundance measurements and the
upper limits, b) possible selection effects, c) boundaries in the parameter
space, d) non-Gaussian errors, e) the intrinsic variety of the objects studied,
and f) abundance systematics. We introduce a Bayesian model to address all
these issues at the same time, thus allowing cross-talk (covariance). On
simulated data, the Bayesian fit recovers the input enrichment history, unlike
in standard analysis. After accounting for a possible dependence on X-ray
temperature, for metal abundance systematics, and for the intrinsic variety of
studied objects, we found that the present-day metal content is not reached
either at high or at low redshifts, but gradually over time: iron abundance
increases by a factor 1.5 in the 7 Gyr sampled by the data. Therefore, feedback
in metal abundance does not end at high redshift. Evolution is established with
a moderate amount of evidence, 19 to 1 odds against faster or slower metal
enrichment histories. We quantify, for the first time, the intrinsic spread in
metal abundance, 18+/-3 %, after correcting for the effect of evolution, X-ray
temperature, and metal abundance systematics. Finally, we also present an
analytic approximation of the X-ray temperature and metal abundance likelihood
functions, which are useful for other regression fitting involving these
parameters. The data for the 130 clusters and code used for the stochastic
computation are provided with the paper.
Combined constraints on intergalactic dust from quasar colours and the soft X-ray background. (arXiv:1209.0553v1 [astro-ph.CO])
Combined constraints on intergalactic dust from quasar colours and the soft X-ray background. (arXiv:1209.0553v1 [astro-ph.CO]):
Unless properly corrected for, the existence of intergalactic dust will
introduce a redshift dependent magnitude offset to standard candle sources.
This would lead to overestimated luminosity distances compared to a dust-free
universe and bias the cosmological parameter estimation as derived from, e.g.,
Type Ia supernovae observations. In this paper, we model the optical extinction
and X-ray scattering properties of intergalactic dust grains to constrain the
intergalactic opacity using a combined analysis of observed quasar colours and
the soft X-ray background. Quasar colours effectively constrain the amount of
intergalactic dust grains smaller than ~0.2 microns, to the point where we
expect the corresponding systematic error in the Type Ia supernova
magnitude-redshift relation to be sub-dominant. Soft X-ray background
observations are helpful in improving the constraints on very large dust grains
for which the amount of optical reddening is very small and therefore is more
difficult to correct for. Our current upper limit corresponds to ~0.25
magnitude dimming at optical wavelengths for a source at redshift z = 1, which
is too small to alleviate the need for dark energy but large in terms of
relative error. However, we expect it to be possible to lower this bound
considerably with an improved understanding of the possible sources of the
X-ray background, in combination with observations of compact X-ray sources
such as Active Galactic Nuclei.
Unless properly corrected for, the existence of intergalactic dust will
introduce a redshift dependent magnitude offset to standard candle sources.
This would lead to overestimated luminosity distances compared to a dust-free
universe and bias the cosmological parameter estimation as derived from, e.g.,
Type Ia supernovae observations. In this paper, we model the optical extinction
and X-ray scattering properties of intergalactic dust grains to constrain the
intergalactic opacity using a combined analysis of observed quasar colours and
the soft X-ray background. Quasar colours effectively constrain the amount of
intergalactic dust grains smaller than ~0.2 microns, to the point where we
expect the corresponding systematic error in the Type Ia supernova
magnitude-redshift relation to be sub-dominant. Soft X-ray background
observations are helpful in improving the constraints on very large dust grains
for which the amount of optical reddening is very small and therefore is more
difficult to correct for. Our current upper limit corresponds to ~0.25
magnitude dimming at optical wavelengths for a source at redshift z = 1, which
is too small to alleviate the need for dark energy but large in terms of
relative error. However, we expect it to be possible to lower this bound
considerably with an improved understanding of the possible sources of the
X-ray background, in combination with observations of compact X-ray sources
such as Active Galactic Nuclei.
The Demographics of Broad Line Quasars in the Mass-Luminosity Plane II. Black Hole Mass and Eddington Ratio Functions. (arXiv:1209.0477v1 [astro-ph.CO])
The Demographics of Broad Line Quasars in the Mass-Luminosity Plane II. Black Hole Mass and Eddington Ratio Functions. (arXiv:1209.0477v1 [astro-ph.CO]):
We employ a flexible Bayesian technique to estimate the black hole mass and
Eddington ratio functions for Type 1 (i.e., broad line) quasars from a
uniformly-selected data set of ~58,000 quasars from the SDSS DR7. We find that
the SDSS becomes significantly incomplete at M_{BH} < 3 x 10^8 M_{Sun} or L /
L_{Edd} < 0.07, and that the number densities of Type 1 quasars continue to
increase down to these limits. Both the mass and Eddington ratio functions show
evidence of downsizing, with the most massive and highest Eddington ratio black
holes experiencing Type 1 quasar phases first, although the Eddington ratio
number densities are flat at z < 2. We estimate the maximum Eddington ratio of
Type 1 quasars in the observable Universe to be L / L_{Edd} ~ 3. Consistent
with our results in Paper I, we do not find statistical evidence for a
so-called "sub-Eddington boundary" in the mass-luminosity plane of broad line
quasars, and demonstrate that such an apparent boundary in the observed
distribution can be caused by selection effect and errors in virial BH mass
estimates. Based on the typical Eddington ratio in a given mass bin, we
estimate typical growth times for the black holes in Type 1 quasars and find
that they are typically comparable to or longer than the age of the universe,
implying an earlier phase of accelerated (i.e., with higher Eddington ratios)
and possibly obscured growth. The large masses probed by our sample imply that
most of our black holes reside in what are locally early type galaxies, and we
interpret our results within the context of models of self-regulated black hole
growth.
We employ a flexible Bayesian technique to estimate the black hole mass and
Eddington ratio functions for Type 1 (i.e., broad line) quasars from a
uniformly-selected data set of ~58,000 quasars from the SDSS DR7. We find that
the SDSS becomes significantly incomplete at M_{BH} < 3 x 10^8 M_{Sun} or L /
L_{Edd} < 0.07, and that the number densities of Type 1 quasars continue to
increase down to these limits. Both the mass and Eddington ratio functions show
evidence of downsizing, with the most massive and highest Eddington ratio black
holes experiencing Type 1 quasar phases first, although the Eddington ratio
number densities are flat at z < 2. We estimate the maximum Eddington ratio of
Type 1 quasars in the observable Universe to be L / L_{Edd} ~ 3. Consistent
with our results in Paper I, we do not find statistical evidence for a
so-called "sub-Eddington boundary" in the mass-luminosity plane of broad line
quasars, and demonstrate that such an apparent boundary in the observed
distribution can be caused by selection effect and errors in virial BH mass
estimates. Based on the typical Eddington ratio in a given mass bin, we
estimate typical growth times for the black holes in Type 1 quasars and find
that they are typically comparable to or longer than the age of the universe,
implying an earlier phase of accelerated (i.e., with higher Eddington ratios)
and possibly obscured growth. The large masses probed by our sample imply that
most of our black holes reside in what are locally early type galaxies, and we
interpret our results within the context of models of self-regulated black hole
growth.
The Black Hole Spin and Soft X-ray Excess of the Luminous Seyfert Galaxy Fairall 9. (arXiv:1209.0468v1 [astro-ph.HE])
The Black Hole Spin and Soft X-ray Excess of the Luminous Seyfert Galaxy Fairall 9. (arXiv:1209.0468v1 [astro-ph.HE]):
We present an analysis of all XMM-Newton and Suzaku X-ray spectra of the
nearby luminous Seyfert galaxy Fairall 9. Confirming previous analyses, we find
robust evidence for a broad iron line associated with X-ray reflection from the
innermost accretion disk. By fitting a spectral model that includes a
relativistically ionized reflection component, we examine the constraints on
the inclination of the inner accretion disk and the black hole spin, and the
complications introduced by the presence of a photoionized emission line
system. Employing multi-epoch fitting, we attempt to obtain robust and
concordant measures of the accretion disk parameters. We also clearly see a
soft X-ray excess in Fairall 9. During certain epochs, the soft excess can be
described with same disk reflection component that produces the iron line.
However, there are epochs where an additional soft component is required. This
can be attributed to either an additional highly-ionized, strongly blurred disk
reflection component, or a new X-ray continuum component.
We present an analysis of all XMM-Newton and Suzaku X-ray spectra of the
nearby luminous Seyfert galaxy Fairall 9. Confirming previous analyses, we find
robust evidence for a broad iron line associated with X-ray reflection from the
innermost accretion disk. By fitting a spectral model that includes a
relativistically ionized reflection component, we examine the constraints on
the inclination of the inner accretion disk and the black hole spin, and the
complications introduced by the presence of a photoionized emission line
system. Employing multi-epoch fitting, we attempt to obtain robust and
concordant measures of the accretion disk parameters. We also clearly see a
soft X-ray excess in Fairall 9. During certain epochs, the soft excess can be
described with same disk reflection component that produces the iron line.
However, there are epochs where an additional soft component is required. This
can be attributed to either an additional highly-ionized, strongly blurred disk
reflection component, or a new X-ray continuum component.
Tracking Down the Source Population Responsible for the Unresolved Cosmic 6-8 keV Background. (arXiv:1209.0467v1 [astro-ph.CO])
Tracking Down the Source Population Responsible for the Unresolved Cosmic 6-8 keV Background. (arXiv:1209.0467v1 [astro-ph.CO]):
Using the 4 Ms Chandra Deep Field-South (CDF-S) survey, we have identified a
sample of 6845 X-ray undetected galaxies that dominates the unresolved ~ 20-25%
of the 6-8 keV cosmic X-ray background (XRB). This sample was constructed by
applying mass and color cuts to sources from a parent catalog based on
GOODS-South HST z-band imaging of the central 6'-radius area of the 4 Ms CDF-S.
The stacked 6-8 keV detection is significant at the 3.9 sigma level, but the
stacked emission was not detected in the 4-6 keV band which indicates the
existence of an underlying population of highly obscured active galactic nuclei
(AGNs). Further examinations of these 6845 galaxies indicate that the galaxies
on the top of the blue cloud and with redshifts of 1 < z < 3, magnitudes of 25
< z_850 < 28, and stellar masses of 2E8 < M_star/M_sun < 2E9 make the majority
contributions to the unresolved 6-8 keV XRB. Such a population is seemingly
surprising given that the majority of the X-ray detected AGNs reside in massive
(> ~1E10 M_sun) galaxies. We discuss constraints upon this underlying AGN
population, supporting evidence for relatively low-mass galaxies hosting highly
obscured AGNs, and prospects for further boosting the stacked signal.
Using the 4 Ms Chandra Deep Field-South (CDF-S) survey, we have identified a
sample of 6845 X-ray undetected galaxies that dominates the unresolved ~ 20-25%
of the 6-8 keV cosmic X-ray background (XRB). This sample was constructed by
applying mass and color cuts to sources from a parent catalog based on
GOODS-South HST z-band imaging of the central 6'-radius area of the 4 Ms CDF-S.
The stacked 6-8 keV detection is significant at the 3.9 sigma level, but the
stacked emission was not detected in the 4-6 keV band which indicates the
existence of an underlying population of highly obscured active galactic nuclei
(AGNs). Further examinations of these 6845 galaxies indicate that the galaxies
on the top of the blue cloud and with redshifts of 1 < z < 3, magnitudes of 25
< z_850 < 28, and stellar masses of 2E8 < M_star/M_sun < 2E9 make the majority
contributions to the unresolved 6-8 keV XRB. Such a population is seemingly
surprising given that the majority of the X-ray detected AGNs reside in massive
(> ~1E10 M_sun) galaxies. We discuss constraints upon this underlying AGN
population, supporting evidence for relatively low-mass galaxies hosting highly
obscured AGNs, and prospects for further boosting the stacked signal.
Tuesday, September 4, 2012
Shocks, Seyferts and the SNR connection: a Chandra observation of the Circinus galaxy. (arXiv:1209.0348v1 [astro-ph.CO])
Shocks, Seyferts and the SNR connection: a Chandra observation of the Circinus galaxy. (arXiv:1209.0348v1 [astro-ph.CO]):
We analyse new Chandra observations of the nearest (D=4 Mpc) Seyfert 2 active
galaxy, Circinus, and match them to pre-existing radio, infrared and optical
data to study the kpc-scale emission. The proximity of Circinus allows us to
observe in striking detail the structure of the radio lobes, revealing for the
first time edge-brightened emission both in X-rays and radio. After considering
various other possible scenarios, we show that this extended emission in
Circinus is most likely caused by a jet-driven outflow, which is driving shells
of strongly shocked gas into the halo of the host galaxy. In this context, we
estimate Mach numbers M=2.7-3.6 and M=2.8-5.3 for the W and E shells
respectively. We derive temperatures of 0.74 (+0.06, -0.05) keV and 0.8-1.8 keV
for the W and E shells, and an expansion velocity of ~900-950 km/s. We estimate
that the total energy (thermal and kinetic) involved in creating both shells is
~2x10^55 erg, and their age is ~10^6 years. Comparing these results with those
we previously obtained for Centaurus A, NGC 3801 and Mrk 6, we show that these
parameters scale approximately with the radio power of the parent AGN. The
spatial coincidence between the X-ray and edge-brightened radio emission in
Circinus resembles the morphology of some SNR shocks. This parallel has been
expected for AGN, but has never been observed before. We investigate what
underlying mechanisms both types of systems may have in common, arguing that,
in Circinus, the edge-brightening in the shells may be accounted for by a B
field enhancement caused by shock compression, but do not preclude some local
particle acceleration. These results can be extrapolated to other low-power
systems, particularly those with late type hosts.
We analyse new Chandra observations of the nearest (D=4 Mpc) Seyfert 2 active
galaxy, Circinus, and match them to pre-existing radio, infrared and optical
data to study the kpc-scale emission. The proximity of Circinus allows us to
observe in striking detail the structure of the radio lobes, revealing for the
first time edge-brightened emission both in X-rays and radio. After considering
various other possible scenarios, we show that this extended emission in
Circinus is most likely caused by a jet-driven outflow, which is driving shells
of strongly shocked gas into the halo of the host galaxy. In this context, we
estimate Mach numbers M=2.7-3.6 and M=2.8-5.3 for the W and E shells
respectively. We derive temperatures of 0.74 (+0.06, -0.05) keV and 0.8-1.8 keV
for the W and E shells, and an expansion velocity of ~900-950 km/s. We estimate
that the total energy (thermal and kinetic) involved in creating both shells is
~2x10^55 erg, and their age is ~10^6 years. Comparing these results with those
we previously obtained for Centaurus A, NGC 3801 and Mrk 6, we show that these
parameters scale approximately with the radio power of the parent AGN. The
spatial coincidence between the X-ray and edge-brightened radio emission in
Circinus resembles the morphology of some SNR shocks. This parallel has been
expected for AGN, but has never been observed before. We investigate what
underlying mechanisms both types of systems may have in common, arguing that,
in Circinus, the edge-brightening in the shells may be accounted for by a B
field enhancement caused by shock compression, but do not preclude some local
particle acceleration. These results can be extrapolated to other low-power
systems, particularly those with late type hosts.
Extreme host galaxy growth in powerful early-epoch radio galaxies. (arXiv:1209.0324v1 [astro-ph.CO])
Extreme host galaxy growth in powerful early-epoch radio galaxies. (arXiv:1209.0324v1 [astro-ph.CO]):
During the first half of the universe's age, a heyday of star-formation must
have occurred because many massive galaxies are in place after that epoch in
cosmic history. Our observations with the revolutionary Herschel Space
Observatory reveal vigorous optically obscured star-formation in the
ultra-massive hosts of many powerful high-redshift 3C quasars and radio
galaxies. This symbiotic occurrence of star-formation and black hole driven
activity is in marked contrast to recent results dealing with Herschel
observations of X-ray selected active galaxies. Three archetypal radio
galaxies, at redshifts 1.132,1.575, and 2.474 are presented here, with inferred
star-formation rates of hundreds of solar masses per year. A series of
spectacular coeval AGN/starburst events may have formed these ultra-massive
galaxies and their massive central black holes during their relatively short
lifetimes.
During the first half of the universe's age, a heyday of star-formation must
have occurred because many massive galaxies are in place after that epoch in
cosmic history. Our observations with the revolutionary Herschel Space
Observatory reveal vigorous optically obscured star-formation in the
ultra-massive hosts of many powerful high-redshift 3C quasars and radio
galaxies. This symbiotic occurrence of star-formation and black hole driven
activity is in marked contrast to recent results dealing with Herschel
observations of X-ray selected active galaxies. Three archetypal radio
galaxies, at redshifts 1.132,1.575, and 2.474 are presented here, with inferred
star-formation rates of hundreds of solar masses per year. A series of
spectacular coeval AGN/starburst events may have formed these ultra-massive
galaxies and their massive central black holes during their relatively short
lifetimes.
Jets and gamma-ray emission from isolated accreting black holes. (arXiv:1209.0293v1 [astro-ph.HE])
Jets and gamma-ray emission from isolated accreting black holes. (arXiv:1209.0293v1 [astro-ph.HE]):
The large number of isolated black holes (IBHs) in the Galaxy, estimated to
be 10^8, implies a very high density of 10^-4 pc^-3 and an average distance
between IBHs of 10 pc. Our study shows that the magnetic flux, accumulated on
the horizon of an IBH because of accretion of interstellar matter, allows the
Blandford-Znajeck mechanism to be activated. Thus, electron-positron jets can
be launched. We have performed 2D numerical modelling which allowed the jet
power to be estimated. Their inferred properties make such jets a feasible
electron accelerator which, in molecular clouds, allows electron energy to be
boosted up to 1 PeV. For the conditions expected in molecular clouds the
radiative cooling time should be comparable to the escape time. Thus these
sources can contribute both to the population of unidentified point-like
sources and to the local cosmic ray (CR) electron spectrum. The impact of the
generated electron CRs depends on the diffusion rate inside molecular clouds
(MCs). If the diffusion regime in a MC is similar to Galactic diffusion, the
produced electrons should rapidly escape the cloud and contribute to the
Galactic CR population at very high energies >100 TeV. However, due to the
modest jet luminosity (at the level of 10^35 erg s^-1) and low filling factor
of MC, these sources cannot make a significant contribution to the spectrum of
cosmic ray electrons at lower energies. On the other hand, if the diffusion
within MCs operates at a rate close to the Bohm limit, the CR electrons
escaping from the source should be confined in the cloud, significantly
contributing to the local density of CRs. The IC emission of these
locally-generated CRs may explain the variety of gamma ray spectra detected
from nearby MCs.
The large number of isolated black holes (IBHs) in the Galaxy, estimated to
be 10^8, implies a very high density of 10^-4 pc^-3 and an average distance
between IBHs of 10 pc. Our study shows that the magnetic flux, accumulated on
the horizon of an IBH because of accretion of interstellar matter, allows the
Blandford-Znajeck mechanism to be activated. Thus, electron-positron jets can
be launched. We have performed 2D numerical modelling which allowed the jet
power to be estimated. Their inferred properties make such jets a feasible
electron accelerator which, in molecular clouds, allows electron energy to be
boosted up to 1 PeV. For the conditions expected in molecular clouds the
radiative cooling time should be comparable to the escape time. Thus these
sources can contribute both to the population of unidentified point-like
sources and to the local cosmic ray (CR) electron spectrum. The impact of the
generated electron CRs depends on the diffusion rate inside molecular clouds
(MCs). If the diffusion regime in a MC is similar to Galactic diffusion, the
produced electrons should rapidly escape the cloud and contribute to the
Galactic CR population at very high energies >100 TeV. However, due to the
modest jet luminosity (at the level of 10^35 erg s^-1) and low filling factor
of MC, these sources cannot make a significant contribution to the spectrum of
cosmic ray electrons at lower energies. On the other hand, if the diffusion
within MCs operates at a rate close to the Bohm limit, the CR electrons
escaping from the source should be confined in the cloud, significantly
contributing to the local density of CRs. The IC emission of these
locally-generated CRs may explain the variety of gamma ray spectra detected
from nearby MCs.
Simultaneous X-ray and optical observations of true Type 2 Seyfert galaxies. (arXiv:1209.0274v1 [astro-ph.GA])
Simultaneous X-ray and optical observations of true Type 2 Seyfert galaxies. (arXiv:1209.0274v1 [astro-ph.GA]):
We present the results of a campaign of simultaneous X-ray and optical
observations of `true' Type 2 Seyfert galaxies candidates, i.e. AGN without a
Broad Line Region (BLR). Out of the initial sample composed by 8 sources, one
object, IC1631, was found to be a misclassified starburst galaxy, another,
Q2130-431, does show broad optical lines, while other two, IRAS01428-0404 and
NGC4698, are very likely absorbed by Compton-thick gas along the line of sight.
Therefore, these four sources are not unabsorbed Seyfert 2s as previously
suggested in the literature. On the other hand, we confirm that NGC3147,
NGC3660, and Q2131-427 belong to the class of true Type 2 Seyfert galaxies,
since they do not show any evidence for a broad component of the optical lines
nor for obscuration in their X-ray spectra. These three sources have low
accretion rates ($\dot{m}= L_{bol}/L_{Edd} \la0.01$), in agreement with
theoretical models which predict that the BLR disappears below a critical value
of $L_{bol}/L_{Edd}$. The last source, Mrk273x, would represent an exception
even of this accretion-dependent versions of the Unification Models, due to its
high X-ray luminosity and accretion rate, and no evidence for obscuration.
However, its optical classification as a Seyfert 2 is only based on the absence
of a broad component of the H\beta, due to the lack of optical spectra
encompassing the H\alpha\ band.
We present the results of a campaign of simultaneous X-ray and optical
observations of `true' Type 2 Seyfert galaxies candidates, i.e. AGN without a
Broad Line Region (BLR). Out of the initial sample composed by 8 sources, one
object, IC1631, was found to be a misclassified starburst galaxy, another,
Q2130-431, does show broad optical lines, while other two, IRAS01428-0404 and
NGC4698, are very likely absorbed by Compton-thick gas along the line of sight.
Therefore, these four sources are not unabsorbed Seyfert 2s as previously
suggested in the literature. On the other hand, we confirm that NGC3147,
NGC3660, and Q2131-427 belong to the class of true Type 2 Seyfert galaxies,
since they do not show any evidence for a broad component of the optical lines
nor for obscuration in their X-ray spectra. These three sources have low
accretion rates ($\dot{m}= L_{bol}/L_{Edd} \la0.01$), in agreement with
theoretical models which predict that the BLR disappears below a critical value
of $L_{bol}/L_{Edd}$. The last source, Mrk273x, would represent an exception
even of this accretion-dependent versions of the Unification Models, due to its
high X-ray luminosity and accretion rate, and no evidence for obscuration.
However, its optical classification as a Seyfert 2 is only based on the absence
of a broad component of the H\beta, due to the lack of optical spectra
encompassing the H\alpha\ band.
Constraining the nature of the Galactic center black hole Sgr A* with present and future observations. (arXiv:1209.0251v1 [astro-ph.HE])
Constraining the nature of the Galactic center black hole Sgr A* with present and future observations. (arXiv:1209.0251v1 [astro-ph.HE]):
The Galactic center is an ideal laboratory to study strong-field general
relativistic phenomena, as the supermassive black hole Sgr A* has the biggest
angular Schwarzschild radius among all black holes. This article presents three
different ways of using the immediate surroundings of Sgr A* as a probe in
order to either constrain its spin, or even test the very nature of this
compact object.
The Galactic center is an ideal laboratory to study strong-field general
relativistic phenomena, as the supermassive black hole Sgr A* has the biggest
angular Schwarzschild radius among all black holes. This article presents three
different ways of using the immediate surroundings of Sgr A* as a probe in
order to either constrain its spin, or even test the very nature of this
compact object.
Supernovae and AGN driven galactic outflows. (arXiv:1209.0242v1 [astro-ph.CO])
Supernovae and AGN driven galactic outflows. (arXiv:1209.0242v1 [astro-ph.CO]):
We present analytical solutions for winds from galaxies with NFW dark matter
halo. We consider winds driven by energy and mass injection from multiple
supernovae, as well as momentum injection due to radiation from a central black
hole. We find that the wind dynamics depends on three velocity scales: (a)
v_star \sim (\dot{E} / 2 \dot{M})^{1/2} describes the effect of starburst
activity, with \dot{E}, \dot{M} as energy and mass injection rate in a central
region of radius R; (b) \vbh ~ (G\mbh / 2 R)^{1/2} for the effect of a central
black hole of mass \mbh on gas at distance R and (c) v_{s} =(GM_h/ 2Cr_s)^{1/2}
which is closely related to the virial velocity, with C as a function of halo
concentration parameter. We find the wind terminal speed to be 2 (v_star^2
+1.5(\Gamma -1) \vbh^2 -v_s^2)^{1/2}, where \Gamma is the ratio of force due to
radiation pressure to gravity of the central black hole. We also find that: (a)
winds from quiescent star forming galaxies cannot escape from 10^{11.5} \le M_h
\le 10^{12.5}Msun galaxies, (b) circumgalactic gas at large distances should be
present for galaxies in this mass range, (c) for an escaping wind, the wind
speed in low to intermediate mass galaxies is ~ 400--1000 km/s, consistent with
observed X-ray temperatures; (d) winds from massive galaxies with AGN have
speeds \gtrsim 1000 km/s. We also find that the ratio [2 v_star ^2 -(1 -\Gamma)
\vbh^2]/ \vvir^2 dictates the amount of gas lost. Used in conjunction with an
appropriate relation between \mbh and M_h, and an appropriate opacity of dust
grains in IR (K band), this ratio becomes minimum at a certain halo mass scale
(M_h ~ 10^{12--12.5} Msun) that signifies the cross-over of AGN domination in
outflow properties from starburst activity at lower masses. We find that
stellar mass for massive galaxies scales as M_star \propto M_h^{0.26},and for
low mass galaxies, M_star \propto M_h^{5/3}.
We present analytical solutions for winds from galaxies with NFW dark matter
halo. We consider winds driven by energy and mass injection from multiple
supernovae, as well as momentum injection due to radiation from a central black
hole. We find that the wind dynamics depends on three velocity scales: (a)
v_star \sim (\dot{E} / 2 \dot{M})^{1/2} describes the effect of starburst
activity, with \dot{E}, \dot{M} as energy and mass injection rate in a central
region of radius R; (b) \vbh ~ (G\mbh / 2 R)^{1/2} for the effect of a central
black hole of mass \mbh on gas at distance R and (c) v_{s} =(GM_h/ 2Cr_s)^{1/2}
which is closely related to the virial velocity, with C as a function of halo
concentration parameter. We find the wind terminal speed to be 2 (v_star^2
+1.5(\Gamma -1) \vbh^2 -v_s^2)^{1/2}, where \Gamma is the ratio of force due to
radiation pressure to gravity of the central black hole. We also find that: (a)
winds from quiescent star forming galaxies cannot escape from 10^{11.5} \le M_h
\le 10^{12.5}Msun galaxies, (b) circumgalactic gas at large distances should be
present for galaxies in this mass range, (c) for an escaping wind, the wind
speed in low to intermediate mass galaxies is ~ 400--1000 km/s, consistent with
observed X-ray temperatures; (d) winds from massive galaxies with AGN have
speeds \gtrsim 1000 km/s. We also find that the ratio [2 v_star ^2 -(1 -\Gamma)
\vbh^2]/ \vvir^2 dictates the amount of gas lost. Used in conjunction with an
appropriate relation between \mbh and M_h, and an appropriate opacity of dust
grains in IR (K band), this ratio becomes minimum at a certain halo mass scale
(M_h ~ 10^{12--12.5} Msun) that signifies the cross-over of AGN domination in
outflow properties from starburst activity at lower masses. We find that
stellar mass for massive galaxies scales as M_star \propto M_h^{0.26},and for
low mass galaxies, M_star \propto M_h^{5/3}.
The jet feedback mechanism (JFM): from supernovae to clusters of galaxies. (arXiv:1209.0168v1 [astro-ph.SR])
The jet feedback mechanism (JFM): from supernovae to clusters of galaxies. (arXiv:1209.0168v1 [astro-ph.SR]):
We study the similarities of jet-medium interactions in several quite
different astrophysical systems using 2D and 3D hydrodynamical numerical
simulations, and find many similarities. The systems include cooling flow (CF)
clusters of galaxies, core-collapse supernovae (CCSNe), planetary nebulae
(PNe), and common envelope (CE) evolution. The similarities include hot bubbles
inflated by jets in a bipolar structure, vortices on the sides of the jets,
vortices inside the inflated bubbles, fragmentation of bubbles to two and more
bubbles, and buoyancy of bubbles. The activity in many cases is regulated by a
negative feedback mechanism. Namely, higher accretion rate leads to stronger
jet activity that in turn suppresses the accretion process. After the jets
power decreases the accretion resumes, and the cycle restarts. In the case of
CF in galaxies and clusters of galaxies we also study the accretion process,
which is most likely by cold clumps, i.e., the cold feedback mechanism. In CF
clusters we find that heating of the intra-cluster medium (ICM) is done by
mixing hot shocked jet gas with the ICM, and not by shocks. Our results
strengthen the jet feedback mechanism (JFM) as a common process in many
astrophysical objects.
We study the similarities of jet-medium interactions in several quite
different astrophysical systems using 2D and 3D hydrodynamical numerical
simulations, and find many similarities. The systems include cooling flow (CF)
clusters of galaxies, core-collapse supernovae (CCSNe), planetary nebulae
(PNe), and common envelope (CE) evolution. The similarities include hot bubbles
inflated by jets in a bipolar structure, vortices on the sides of the jets,
vortices inside the inflated bubbles, fragmentation of bubbles to two and more
bubbles, and buoyancy of bubbles. The activity in many cases is regulated by a
negative feedback mechanism. Namely, higher accretion rate leads to stronger
jet activity that in turn suppresses the accretion process. After the jets
power decreases the accretion resumes, and the cycle restarts. In the case of
CF in galaxies and clusters of galaxies we also study the accretion process,
which is most likely by cold clumps, i.e., the cold feedback mechanism. In CF
clusters we find that heating of the intra-cluster medium (ICM) is done by
mixing hot shocked jet gas with the ICM, and not by shocks. Our results
strengthen the jet feedback mechanism (JFM) as a common process in many
astrophysical objects.
Discovery of 100 supernovae among 700,000 Sloan spectra: the Type-Ia supernova rate versus galaxy mass and star-formation rate at redshift ~0.1. (arXiv:1209.0008v1 [astro-ph.CO])
Discovery of 100 supernovae among 700,000 Sloan spectra: the Type-Ia supernova rate versus galaxy mass and star-formation rate at redshift ~0.1. (arXiv:1209.0008v1 [astro-ph.CO]):
Using a method to discover and classify supernovae (SNe) in galaxy spectra,
we find 90 Type Ia SNe (SNe Ia) and 10 Type IIP SNe among the ~700,000 galaxy
spectra in the Sloan Digital Sky Survey Data Release 7 that have VESPA-derived
star-formation histories (SFHs). We use the SN Ia sample to measure SN Ia rates
per unit stellar mass. We confirm, at the median redshift of the sample, z =
0.1, the inverse dependence on galaxy mass of the SN Ia rate per unit mass,
previously reported by Li et al. (2011b) for a local sample. We further
confirm, following Kistler et al. (2011), that this relation can be explained
by the combination of galaxy "downsizing" and a power-law delay-time
distribution (DTD; the distribution of times that elapse between a hypothetical
burst of star formation and the subsequent SN Ia explosions) with an index of
-1, inherent to the double-degenerate progenitor scenario. We use the method of
Maoz et al. (2011) to recover the DTD by comparing the number of SNe Ia hosted
by each galaxy in our sample with the VESPA-derived SFH of the stellar
population within the spectral aperture. In this galaxy sample, which is
dominated by old and massive galaxies, we recover a "delayed" component to the
DTD of 4.5 +/- 0.6 X 10^-14 SNe Msun^-1 yr^-1 for delays in the range > 2.4
Gyr. The mass-normalized SN Ia rate, averaged over all masses and redshifts in
our galaxy sample, is R(Ia,M,z=0.1) = 0.10 +/- 0.01 SNuM, and the volumetric
rate is R(Ia,V,z=0.1) = 0.25 +/- 0.03 X 10^-4 SNe yr^-1 Mpc^-3. This is the
most precise SN Ia rate measurement at this redshift, and is consistent with
rates and the rate evolution from other SN Ia surveys.
Using a method to discover and classify supernovae (SNe) in galaxy spectra,
we find 90 Type Ia SNe (SNe Ia) and 10 Type IIP SNe among the ~700,000 galaxy
spectra in the Sloan Digital Sky Survey Data Release 7 that have VESPA-derived
star-formation histories (SFHs). We use the SN Ia sample to measure SN Ia rates
per unit stellar mass. We confirm, at the median redshift of the sample, z =
0.1, the inverse dependence on galaxy mass of the SN Ia rate per unit mass,
previously reported by Li et al. (2011b) for a local sample. We further
confirm, following Kistler et al. (2011), that this relation can be explained
by the combination of galaxy "downsizing" and a power-law delay-time
distribution (DTD; the distribution of times that elapse between a hypothetical
burst of star formation and the subsequent SN Ia explosions) with an index of
-1, inherent to the double-degenerate progenitor scenario. We use the method of
Maoz et al. (2011) to recover the DTD by comparing the number of SNe Ia hosted
by each galaxy in our sample with the VESPA-derived SFH of the stellar
population within the spectral aperture. In this galaxy sample, which is
dominated by old and massive galaxies, we recover a "delayed" component to the
DTD of 4.5 +/- 0.6 X 10^-14 SNe Msun^-1 yr^-1 for delays in the range > 2.4
Gyr. The mass-normalized SN Ia rate, averaged over all masses and redshifts in
our galaxy sample, is R(Ia,M,z=0.1) = 0.10 +/- 0.01 SNuM, and the volumetric
rate is R(Ia,V,z=0.1) = 0.25 +/- 0.03 X 10^-4 SNe yr^-1 Mpc^-3. This is the
most precise SN Ia rate measurement at this redshift, and is consistent with
rates and the rate evolution from other SN Ia surveys.
The Temperature-Density Relation in the Intergalactic Medium at Redshift =2.4. (arXiv:1209.0005v1 [astro-ph.CO])
The Temperature-Density Relation in the Intergalactic Medium at Redshift <z>=2.4. (arXiv:1209.0005v1 [astro-ph.CO]):
We present new measurements of the temperature-density (T-rho) relation for
neutral hydrogen in the 2.0 < z < 2.8 intergalactic medium (IGM) using a sample
of ~6000 individual HI absorbers fitted with Voigt profiles constrained in all
cases by multiple Lyman series transitions. We find model independent evidence
for a positive correlation between the column density of HI (NHI) and the
minimum observed velocity width of absorbers (bmin). With minimal
interpretation, this implies that the temperature-density relation in the IGM
is not "inverted", contrary to many recent studies. Fitting bmin as a function
of NHI results in line width - column density dependence of the form bdmin =
b_0 [NHI/N_(HI,0)]^(Gamma-1) with a minimum line width at mean density rhobar
[N_(HI, 0) = 10^13.6 cm^-2] of b_0 = 17.9 +- 0.2 km/s and a power-law index of
(Gamma-1) = 0.15 +- 0.02. Using analytic arguments, these measurements imply an
"equation of state" for the IGM at <z>= 2.4 of the form T = T_0
(rho/rhobar)^(gamma-1) with a temperature at mean density of T_0 = (1.94 +-
0.05) x 10^4 K and a power-law index (gamma -1) = 0.46 +- 0.05.
We present new measurements of the temperature-density (T-rho) relation for
neutral hydrogen in the 2.0 < z < 2.8 intergalactic medium (IGM) using a sample
of ~6000 individual HI absorbers fitted with Voigt profiles constrained in all
cases by multiple Lyman series transitions. We find model independent evidence
for a positive correlation between the column density of HI (NHI) and the
minimum observed velocity width of absorbers (bmin). With minimal
interpretation, this implies that the temperature-density relation in the IGM
is not "inverted", contrary to many recent studies. Fitting bmin as a function
of NHI results in line width - column density dependence of the form bdmin =
b_0 [NHI/N_(HI,0)]^(Gamma-1) with a minimum line width at mean density rhobar
[N_(HI, 0) = 10^13.6 cm^-2] of b_0 = 17.9 +- 0.2 km/s and a power-law index of
(Gamma-1) = 0.15 +- 0.02. Using analytic arguments, these measurements imply an
"equation of state" for the IGM at <z>= 2.4 of the form T = T_0
(rho/rhobar)^(gamma-1) with a temperature at mean density of T_0 = (1.94 +-
0.05) x 10^4 K and a power-law index (gamma -1) = 0.46 +- 0.05.
Monday, September 3, 2012
Radiative properties of magnetic neutron stars with metallic surfaces and thin atmospheres. (arXiv:1208.6582v1 [astro-ph.HE])
Radiative properties of magnetic neutron stars with metallic surfaces and thin atmospheres. (arXiv:1208.6582v1 [astro-ph.HE]):
The goal of this work is to develop a simple analytic description of the
emission properties (spectrum and polarization) of the condensed, strongly
magnetized surface of neutron stars. We have improved the method of van
Adelsberg et al. (2005) (arXiv:astro-ph/0406001) for calculating the spectral
properties of condensed magnetized surfaces. Using the improved method, we
calculate the reflectivity of an iron surface at magnetic field strengths B
\sim (10^{12} - 10^{14}) G, with various inclinations of the magnetic field
lines and radiation beam with respect to the surface and each other. We
construct analytic expressions for the emissivity of this surface as functions
of the photon energy, magnetic field strength, and the three angles that
determine the geometry of the local problem. Using these expressions, we
calculate X-ray spectra for neutron stars with condensed iron surfaces covered
by thin partially ionized hydrogen atmospheres. We develop simple analytic
descriptions of the intensity and polarization of radiation emitted or
reflected by condensed iron surfaces of neutron stars with strong magnetic
fields typical for isolated neutron stars. This description provides boundary
conditions at the bottom of a thin atmosphere, which are more accurate than
previously used approximations. The spectra calculated with this improvement
show absorption features different from those in simplified models. The
approach developed in this paper yields results that can facilitate modeling
and interpretation of the X-ray spectra of isolated, strongly magnetized,
thermally emitting neutron stars.
The goal of this work is to develop a simple analytic description of the
emission properties (spectrum and polarization) of the condensed, strongly
magnetized surface of neutron stars. We have improved the method of van
Adelsberg et al. (2005) (arXiv:astro-ph/0406001) for calculating the spectral
properties of condensed magnetized surfaces. Using the improved method, we
calculate the reflectivity of an iron surface at magnetic field strengths B
\sim (10^{12} - 10^{14}) G, with various inclinations of the magnetic field
lines and radiation beam with respect to the surface and each other. We
construct analytic expressions for the emissivity of this surface as functions
of the photon energy, magnetic field strength, and the three angles that
determine the geometry of the local problem. Using these expressions, we
calculate X-ray spectra for neutron stars with condensed iron surfaces covered
by thin partially ionized hydrogen atmospheres. We develop simple analytic
descriptions of the intensity and polarization of radiation emitted or
reflected by condensed iron surfaces of neutron stars with strong magnetic
fields typical for isolated neutron stars. This description provides boundary
conditions at the bottom of a thin atmosphere, which are more accurate than
previously used approximations. The spectra calculated with this improvement
show absorption features different from those in simplified models. The
approach developed in this paper yields results that can facilitate modeling
and interpretation of the X-ray spectra of isolated, strongly magnetized,
thermally emitting neutron stars.
The ATLAS 5.5 GHz survey of the Extended Chandra Deep Field South: Catalogue, Source Counts and Spectral Indices. (arXiv:1208.6306v1 [astro-ph.CO])
The ATLAS 5.5 GHz survey of the Extended Chandra Deep Field South: Catalogue, Source Counts and Spectral Indices. (arXiv:1208.6306v1 [astro-ph.CO]):
Star forming galaxies are thought to dominate the sub-mJy radio population,
but recent work has shown that low luminosity AGN can still make a significant
contribution to the faint radio source population. Spectral indices are an
important tool for understanding the emission mechanism of the faint radio
sources. We have observed the extended Chandra Deep Field South at 5.5 GHz
using a mosaic of 42 pointings with the Australia Telescope Compact Array
(ATCA). Our image reaches an almost uniform sensitivity of ~12 microJy rms over
0.25 deg^2 with a restoring beam of 4.9 x 2.0 arcsec, making it one of the
deepest 6cm surveys to date. We present the 5.5 GHz catalogue and source counts
from this field. We take advantage of the large amounts of ancillary data in
this field to study the 1.4 to 5.5 GHz spectral indices of the sub-mJy
population. For the full 5.5 GHz selected sample we find a flat median spectral
index, alpha_med = -0.40, which is consistent with previous results. However,
the spectral index appears to steepen at the faintest flux density levels
(S_{5.5 GHz} < 0.1 mJy), where alpha_med = -0.68. We performed stacking
analysis of the faint 1.4 GHz selected sample (40 < S_{1.4 GHz} < 200 microJy)
and also find a steep average spectral index, alpha = -0.8, consistent with
synchrotron emission. We find a weak trend of steepening spectral index with
redshift. Several young AGN candidates are identified using spectral indices,
suggesting Gigahertz Peaked Spectrum (GPS) sources are as common in the mJy
population as they are at Jy levels.
Star forming galaxies are thought to dominate the sub-mJy radio population,
but recent work has shown that low luminosity AGN can still make a significant
contribution to the faint radio source population. Spectral indices are an
important tool for understanding the emission mechanism of the faint radio
sources. We have observed the extended Chandra Deep Field South at 5.5 GHz
using a mosaic of 42 pointings with the Australia Telescope Compact Array
(ATCA). Our image reaches an almost uniform sensitivity of ~12 microJy rms over
0.25 deg^2 with a restoring beam of 4.9 x 2.0 arcsec, making it one of the
deepest 6cm surveys to date. We present the 5.5 GHz catalogue and source counts
from this field. We take advantage of the large amounts of ancillary data in
this field to study the 1.4 to 5.5 GHz spectral indices of the sub-mJy
population. For the full 5.5 GHz selected sample we find a flat median spectral
index, alpha_med = -0.40, which is consistent with previous results. However,
the spectral index appears to steepen at the faintest flux density levels
(S_{5.5 GHz} < 0.1 mJy), where alpha_med = -0.68. We performed stacking
analysis of the faint 1.4 GHz selected sample (40 < S_{1.4 GHz} < 200 microJy)
and also find a steep average spectral index, alpha = -0.8, consistent with
synchrotron emission. We find a weak trend of steepening spectral index with
redshift. Several young AGN candidates are identified using spectral indices,
suggesting Gigahertz Peaked Spectrum (GPS) sources are as common in the mJy
population as they are at Jy levels.
Saturday, September 1, 2012
Spin Evolution of Supermassive Black Holes and Galactic Nuclei. (arXiv:1208.6274v1 [astro-ph.GA])
Spin Evolution of Supermassive Black Holes and Galactic Nuclei. (arXiv:1208.6274v1 [astro-ph.GA]):
The spin angular momentum S of a supermassive black hole (SBH) precesses due
to torques from orbiting stars, and the stellar orbits precess due to dragging
of inertial frames by the spinning hole. We solve the coupled post-Newtonian
equations describing the joint evolution of S and the stellar angular momenta
Lj, j = 1...N in spherical, rotating nuclear star clusters. In the absence of
gravitational interactions between the stars, two evolutionary modes are found:
(1) nearly uniform precession of S about the total angular momentum vector of
the system; (2) damped precession, leading, in less than one precessional
period, to alignment of S with the angular momentum of the rotating cluster.
Beyond a certain distance from the SBH, the time scale for angular momentum
changes due to gravitational encounters between the stars is shorter than
spin-orbit precession times. We present a model, based on the
Ornstein-Uhlenbeck equation, for the stochastic evolution of star clusters due
to gravitational encounters and use it to evaluate the evolution of S in nuclei
where changes in the Lj are due to frame dragging close to the SBH and to
encounters farther out. Long-term evolution in this case is well described as
uniform precession of the SBH about the cluster's rotational axis, with an
increasingly important stochastic contribution when SBH masses are small. Spin
precessional periods are predicted to be strongly dependent on nuclear
properties, but typical values are 10-100 Myr for low-mass SBHs in dense
nuclei, 100 Myr - 10 Gyr for intermediate mass SBHs, and > 10 Gyr for the most
massive SBHs. We compare the evolution of SBH spins in stellar nuclei to the
case of torquing by an inclined, gaseous accretion disk.
The spin angular momentum S of a supermassive black hole (SBH) precesses due
to torques from orbiting stars, and the stellar orbits precess due to dragging
of inertial frames by the spinning hole. We solve the coupled post-Newtonian
equations describing the joint evolution of S and the stellar angular momenta
Lj, j = 1...N in spherical, rotating nuclear star clusters. In the absence of
gravitational interactions between the stars, two evolutionary modes are found:
(1) nearly uniform precession of S about the total angular momentum vector of
the system; (2) damped precession, leading, in less than one precessional
period, to alignment of S with the angular momentum of the rotating cluster.
Beyond a certain distance from the SBH, the time scale for angular momentum
changes due to gravitational encounters between the stars is shorter than
spin-orbit precession times. We present a model, based on the
Ornstein-Uhlenbeck equation, for the stochastic evolution of star clusters due
to gravitational encounters and use it to evaluate the evolution of S in nuclei
where changes in the Lj are due to frame dragging close to the SBH and to
encounters farther out. Long-term evolution in this case is well described as
uniform precession of the SBH about the cluster's rotational axis, with an
increasingly important stochastic contribution when SBH masses are small. Spin
precessional periods are predicted to be strongly dependent on nuclear
properties, but typical values are 10-100 Myr for low-mass SBHs in dense
nuclei, 100 Myr - 10 Gyr for intermediate mass SBHs, and > 10 Gyr for the most
massive SBHs. We compare the evolution of SBH spins in stellar nuclei to the
case of torquing by an inclined, gaseous accretion disk.
Disentangling AGN and Star Formation in Soft X-rays. (arXiv:1208.6233v1 [astro-ph.CO])
Disentangling AGN and Star Formation in Soft X-rays. (arXiv:1208.6233v1 [astro-ph.CO]):
We have explored the interplay of star formation and AGN activity in soft
X-rays (0.5-2 keV) in two samples of Seyfert 2 galaxies (Sy2s). Using a
combination of low resolution CCD spectra from Chandra and XMM-Newton, we
modeled the soft emission of 34 Sy2s using power law and thermal models. For
the 11 sources with high signal-to-noise Chandra imaging of the diffuse host
galaxy emission, we estimate the luminosity due to star formation by removing
the AGN, fitting the residual emission. The AGN and star formation
contributions to the soft X-ray luminosity (i.e. L$_{x,AGN}$ and L$_{x,SF}$)
for the remaining 24 Sy2s were estimated from the power law and thermal
luminosities derived from spectral fitting. These luminosities were scaled
based on a template derived from XSINGS analysis of normal star forming
galaxies. To account for errors in the luminosities derived from spectral
fitting and the spread in the scaling factor, we estimated L$_{x,AGN}$ and
L$_{x,SF}$ from Monte Carlo simulations. These simulated luminosities agree
with L$_{x,AGN}$ and L$_{x,SF}$ derived from Chandra imaging analysis within a
3\sigma\ confidence level. Using the infrared [NeII]12.8\mu m and [OIV]26\mu m
lines as a proxy of star formation and AGN activity, respectively, we
independently disentangle the contributions of these two processes to the total
soft X-ray emission. This decomposition generally agrees with L$_{x,SF}$ and
L$_{x,AGN}$ at the 3\sigma\ level. In the absence of resolvable nuclear
emission, our decomposition method provides a reasonable estimate of emission
due to star formation in galaxies hosting type 2 AGN.
We have explored the interplay of star formation and AGN activity in soft
X-rays (0.5-2 keV) in two samples of Seyfert 2 galaxies (Sy2s). Using a
combination of low resolution CCD spectra from Chandra and XMM-Newton, we
modeled the soft emission of 34 Sy2s using power law and thermal models. For
the 11 sources with high signal-to-noise Chandra imaging of the diffuse host
galaxy emission, we estimate the luminosity due to star formation by removing
the AGN, fitting the residual emission. The AGN and star formation
contributions to the soft X-ray luminosity (i.e. L$_{x,AGN}$ and L$_{x,SF}$)
for the remaining 24 Sy2s were estimated from the power law and thermal
luminosities derived from spectral fitting. These luminosities were scaled
based on a template derived from XSINGS analysis of normal star forming
galaxies. To account for errors in the luminosities derived from spectral
fitting and the spread in the scaling factor, we estimated L$_{x,AGN}$ and
L$_{x,SF}$ from Monte Carlo simulations. These simulated luminosities agree
with L$_{x,AGN}$ and L$_{x,SF}$ derived from Chandra imaging analysis within a
3\sigma\ confidence level. Using the infrared [NeII]12.8\mu m and [OIV]26\mu m
lines as a proxy of star formation and AGN activity, respectively, we
independently disentangle the contributions of these two processes to the total
soft X-ray emission. This decomposition generally agrees with L$_{x,SF}$ and
L$_{x,AGN}$ at the 3\sigma\ level. In the absence of resolvable nuclear
emission, our decomposition method provides a reasonable estimate of emission
due to star formation in galaxies hosting type 2 AGN.
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