Principal Component Analysis of Spectral Line Data: Analytic Formulation. (arXiv:1305.5071v1 [astro-ph.GA]):
Principal component analysis is a powerful statistical system to investigate
the structure and dynamics of the molecular interstellar medium, with
particular emphasis on the study of turbulence, as revealed by spectroscopic
imaging of molecular line emission. To-date, the method to retrieve the power
law index of the velocity structure function or power spectrum has relied on an
empirical calibration and testing with model turbulent velocity fields, while
lacking a firm theoretical basis. In this paper, we present an analytic
formulation that reveals the detailed mechanics of the method and confirms
previous empirical calibrations of its recovery of the scale dependence of
turbulent velocity fluctuations.
Monday, June 10, 2013
Unveiling the corona of the Milky Way via ram-pressure stripping of dwarf satellites. (arXiv:1305.4176v1 [astro-ph.GA])
Unveiling the corona of the Milky Way via ram-pressure stripping of dwarf satellites. (arXiv:1305.4176v1 [astro-ph.GA]):
The spatial segregation between dSphs and dIrrs in the Local Group has long
been regarded as evidence of an interaction with their host galaxies. In this
paper, we assume that ram-pressure stripping is the dominant mechanism that
removed gas from the dSphs and we use this to derive a lower bound on the
density of the corona of the Milky Way at large distances (50-90 kpc) from the
Galactic centre. At the same time, we derive an upper bound by demanding that
the interstellar medium of the dSphs is in pressure equilibrium with the hot
corona. We consider two dwarfs (Sextans and Carina) with well-determined orbits
and star formation histories. Our approach introduces several novel features:
we use the measured star formation histories of the dwarfs to derive the time
at which they last lost their gas, and (via a modified version of the
Kennicutt-Schmidt relation) their internal gas density at that time; we use a
large suite of 2D hydrodynamical simulations to model the gas stripping; and we
include supernova feedback tied to the gas content. Despite having very
different orbits and star formation histories, we find results for the two
dSphs that are in excellent agreement with one another. We derive an average
particle density of the corona of the Milky Way at 50-90 kpc in the range
1.3-3.6 10^{-4} cm^{-3}. Including additional constraints from X-ray emission
limits and pulsar dispersion measurements, we extrapolate Galactic coronal
density profiles and we estimate the fraction of baryons that can exist within
the virial radius of the Milky Way. For an isothermal corona (T=1.8 10^6 K)
this is small, 10-20 % of the universal baryon fraction. Only a hot (T=3 10^6
K) and adiabatic corona can contain all of the Galaxy's missing baryons. Models
for the Milky Way must explain why its corona is in a hot adiabatic thermal
state or why a large fraction of its baryons lie beyond the virial radius.
The spatial segregation between dSphs and dIrrs in the Local Group has long
been regarded as evidence of an interaction with their host galaxies. In this
paper, we assume that ram-pressure stripping is the dominant mechanism that
removed gas from the dSphs and we use this to derive a lower bound on the
density of the corona of the Milky Way at large distances (50-90 kpc) from the
Galactic centre. At the same time, we derive an upper bound by demanding that
the interstellar medium of the dSphs is in pressure equilibrium with the hot
corona. We consider two dwarfs (Sextans and Carina) with well-determined orbits
and star formation histories. Our approach introduces several novel features:
we use the measured star formation histories of the dwarfs to derive the time
at which they last lost their gas, and (via a modified version of the
Kennicutt-Schmidt relation) their internal gas density at that time; we use a
large suite of 2D hydrodynamical simulations to model the gas stripping; and we
include supernova feedback tied to the gas content. Despite having very
different orbits and star formation histories, we find results for the two
dSphs that are in excellent agreement with one another. We derive an average
particle density of the corona of the Milky Way at 50-90 kpc in the range
1.3-3.6 10^{-4} cm^{-3}. Including additional constraints from X-ray emission
limits and pulsar dispersion measurements, we extrapolate Galactic coronal
density profiles and we estimate the fraction of baryons that can exist within
the virial radius of the Milky Way. For an isothermal corona (T=1.8 10^6 K)
this is small, 10-20 % of the universal baryon fraction. Only a hot (T=3 10^6
K) and adiabatic corona can contain all of the Galaxy's missing baryons. Models
for the Milky Way must explain why its corona is in a hot adiabatic thermal
state or why a large fraction of its baryons lie beyond the virial radius.
Summary of the 2013 IACHEC Meeting. (arXiv:1305.4480v1 [astro-ph.IM])
Summary of the 2013 IACHEC Meeting. (arXiv:1305.4480v1 [astro-ph.IM]):
We present the main results of the 8th International Astronomical Consortium
for High Energy Calibration (IACHEC) meeting, held in Theddingworth,
Leicestershire, between March 25 and 28, 2013. Over 50 scientists directly
involved in the calibration of operational and future high-energy missions
gathered during 3.5 days to discuss the status of the X-ray payload
inter-calibration, as well as possible ways to improve it. Sect. 4 of this
Report summarises our current understanding of the energy-dependent
inter-calibration status.
We present the main results of the 8th International Astronomical Consortium
for High Energy Calibration (IACHEC) meeting, held in Theddingworth,
Leicestershire, between March 25 and 28, 2013. Over 50 scientists directly
involved in the calibration of operational and future high-energy missions
gathered during 3.5 days to discuss the status of the X-ray payload
inter-calibration, as well as possible ways to improve it. Sect. 4 of this
Report summarises our current understanding of the energy-dependent
inter-calibration status.
Asymmetric Ejecta Distribution in SN 1006. (arXiv:1305.4489v1 [astro-ph.HE])
Asymmetric Ejecta Distribution in SN 1006. (arXiv:1305.4489v1 [astro-ph.HE]):
We present the results from deep X-ray observations (~400 ks in total) of SN
1006 by the X-ray astronomy satellite Suzaku. The thermal spectrum from the
entire supernova remnant (SNR) exhibits prominent emission lines of O, Ne, Mg,
Si, S, Ar, Ca, and Fe. The observed abundance pattern in the ejecta components
is in good agreement with that predicted by a standard model of Type Ia
supernovae (SNe). The spatially resolved analysis reveals that the distribution
of the O-burning and incomplete Si-burning products (Si, S, and Ar) is
asymmetric, while that of the C-burning products (O, Ne, and Mg) is relatively
uniform in the SNR interior. The peak position of the former is clearly shifted
by 5' (~3.2 pc at a distance of 2.2 kpc) to the southeast from the SNR's
geometric center. Using the SNR age of ~1000 yr, we constrain the velocity
asymmetry (in projection) of ejecta to be ~3100 km/s. The abundance of Fe is
also significantly higher in the southeast region than in the northwest region.
Given that the non-uniformity is observed only among the heavier elements (Si
through Fe), we argue that SN 1006 originates from an asymmetric explosion, as
is expected from recent multi-dimensional simulations of Type Ia SNe, although
we cannot eliminate the possibility that an inhomogeneous ambient medium
induced the apparent non-uniformity. Possible evidence for the Cr K-shell line
and line broadening in the Fe K-shell emission is also found.
We present the results from deep X-ray observations (~400 ks in total) of SN
1006 by the X-ray astronomy satellite Suzaku. The thermal spectrum from the
entire supernova remnant (SNR) exhibits prominent emission lines of O, Ne, Mg,
Si, S, Ar, Ca, and Fe. The observed abundance pattern in the ejecta components
is in good agreement with that predicted by a standard model of Type Ia
supernovae (SNe). The spatially resolved analysis reveals that the distribution
of the O-burning and incomplete Si-burning products (Si, S, and Ar) is
asymmetric, while that of the C-burning products (O, Ne, and Mg) is relatively
uniform in the SNR interior. The peak position of the former is clearly shifted
by 5' (~3.2 pc at a distance of 2.2 kpc) to the southeast from the SNR's
geometric center. Using the SNR age of ~1000 yr, we constrain the velocity
asymmetry (in projection) of ejecta to be ~3100 km/s. The abundance of Fe is
also significantly higher in the southeast region than in the northwest region.
Given that the non-uniformity is observed only among the heavier elements (Si
through Fe), we argue that SN 1006 originates from an asymmetric explosion, as
is expected from recent multi-dimensional simulations of Type Ia SNe, although
we cannot eliminate the possibility that an inhomogeneous ambient medium
induced the apparent non-uniformity. Possible evidence for the Cr K-shell line
and line broadening in the Fe K-shell emission is also found.
A statistical relation between the X-ray spectral index and Eddington ratio of active galactic nuclei in deep surveys. (arXiv:1305.3917v1 [astro-ph.HE])
A statistical relation between the X-ray spectral index and Eddington ratio of active galactic nuclei in deep surveys. (arXiv:1305.3917v1 [astro-ph.HE]):
We present an investigation into how well the properties of the accretion
flow onto a supermassive black hole may be coupled to those of the overlying
hot corona. To do so, we specifically measure the characteristic spectral
index, Gamma, of a power-law energy distribution, over an energy range of 2 to
10 keV, for X-ray selected, broad-lined radio-quiet AGN up to z~2 in COSMOS and
E-CDF-S. We test the previously reported dependence between Gamma and black
hole mass, FWHM and Eddington ratio using a sample of AGN covering a broad
range in these parameters based on both the Mg ii and H-alpha emission lines
with the later afforded by recent near infrared spectroscopic observations
using Subaru/FMOS. We calculate the Eddington ratios, lambda_Edd, for sources
where a bolometric luminosity (L_Bol) has been presented in the literature,
based on SED fitting, or, for sources where these data do not exist, we
calculate L_Bol using a bolometric correction to the LX, derived from a
relationship between the bolometric correction, and LX/L3000. From a sample of
69 X-ray bright sources (>250 counts), where Gamma can be measured with
greatest precision, with an estimate of L_Bol, we find a statistically
significant correlation between Gamma and lambda_Edd, which is highly
significant with a chance probability of 6.59x10^-8. A statistically
significant correlation between Gamma and the FWHM of the optical lines is
confirmed, but at lower significance than with lambda_Edd indicating that
lambda_Edd is the key parameter driving conditions in the corona. Linear
regression analysis reveals that Gamma=(0.32+/-0.05)log10
lambda_Edd+(2.27+/-0.06) and
Gamma=(-0.69+/-0.11)log10(FWHM/km/s)+(4.44+/-0.42). Our results on
Gamma-lambda_Edd are in very good agreement with previous results. (ABRIDGED)
We present an investigation into how well the properties of the accretion
flow onto a supermassive black hole may be coupled to those of the overlying
hot corona. To do so, we specifically measure the characteristic spectral
index, Gamma, of a power-law energy distribution, over an energy range of 2 to
10 keV, for X-ray selected, broad-lined radio-quiet AGN up to z~2 in COSMOS and
E-CDF-S. We test the previously reported dependence between Gamma and black
hole mass, FWHM and Eddington ratio using a sample of AGN covering a broad
range in these parameters based on both the Mg ii and H-alpha emission lines
with the later afforded by recent near infrared spectroscopic observations
using Subaru/FMOS. We calculate the Eddington ratios, lambda_Edd, for sources
where a bolometric luminosity (L_Bol) has been presented in the literature,
based on SED fitting, or, for sources where these data do not exist, we
calculate L_Bol using a bolometric correction to the LX, derived from a
relationship between the bolometric correction, and LX/L3000. From a sample of
69 X-ray bright sources (>250 counts), where Gamma can be measured with
greatest precision, with an estimate of L_Bol, we find a statistically
significant correlation between Gamma and lambda_Edd, which is highly
significant with a chance probability of 6.59x10^-8. A statistically
significant correlation between Gamma and the FWHM of the optical lines is
confirmed, but at lower significance than with lambda_Edd indicating that
lambda_Edd is the key parameter driving conditions in the corona. Linear
regression analysis reveals that Gamma=(0.32+/-0.05)log10
lambda_Edd+(2.27+/-0.06) and
Gamma=(-0.69+/-0.11)log10(FWHM/km/s)+(4.44+/-0.42). Our results on
Gamma-lambda_Edd are in very good agreement with previous results. (ABRIDGED)
Searching for a 0.1-1 keV Cosmic Axion Background. (arXiv:1305.3603v1 [astro-ph.CO])
Searching for a 0.1-1 keV Cosmic Axion Background. (arXiv:1305.3603v1 [astro-ph.CO]):
Primordial decays of string theory moduli at z \sim 10^{12} naturally
generate a dark radiation Cosmic Axion Background (CAB) with 0.1 - 1 keV
energies. This CAB can be detected through axion-photon conversion in
astrophysical magnetic fields to give quasi-thermal excesses in the extreme
ultraviolet and soft X-ray bands. Substantial and observable luminosities may
be generated even for axion-photon couplings \ll 10^{-11} GeV^{-1}. We propose
that axion-photon conversion may explain the observed excess emission of soft
X-rays from galaxy clusters, and may also contribute to the diffuse unresolved
cosmic X-ray background. We list a number of correlated predictions of the
scenario.
Primordial decays of string theory moduli at z \sim 10^{12} naturally
generate a dark radiation Cosmic Axion Background (CAB) with 0.1 - 1 keV
energies. This CAB can be detected through axion-photon conversion in
astrophysical magnetic fields to give quasi-thermal excesses in the extreme
ultraviolet and soft X-ray bands. Substantial and observable luminosities may
be generated even for axion-photon couplings \ll 10^{-11} GeV^{-1}. We propose
that axion-photon conversion may explain the observed excess emission of soft
X-rays from galaxy clusters, and may also contribute to the diffuse unresolved
cosmic X-ray background. We list a number of correlated predictions of the
scenario.
Sunday, May 19, 2013
Unveiling a population of galaxies harboring low-mass black holes with X-rays. (arXiv:1305.3826v1 [astro-ph.CO])
Unveiling a population of galaxies harboring low-mass black holes with X-rays. (arXiv:1305.3826v1 [astro-ph.CO]):
We report the discovery of three low-mass black hole candidates residing in
the centers of low-mass galaxies at z<0.3 in the Chandra Deep Field - South
Survey. These black holes are initially identified as candidate active galactic
nuclei based on their X-ray emission in deep Chandra observations.
Multi-wavelength observations are used to strengthen our claim that such
emission is powered by an accreting supermassive black hole. While the X-ray
luminosities are low at L_X ~ 10^40 erg s^-1 (and variable in one case), we
argue that they are unlikely to be attributed to star formation based on
H\alpha or UV-fluxes. Optical spectroscopy with Keck/DEIMOS and VLT/FORS allows
us to (1) measure accurate redshifts, (2) confirm their low stellar host mass,
(3) investigate the source(s) of photo-ionization, and (4) estimate extinction.
With stellar masses of M* < 3*10^9 M_\sun determined from HST/ACS imaging, the
host galaxies are among the lowest mass systems known to host actively
accreting black holes. We estimate BH masses M_BH ~ 2*10^5 M_\sun based on
scaling relations between BH mass and host properties for more luminous
systems. In one case, a broad component of the H\alpha emission-line profile is
detected thus providing a virial mass estimate. Black holes in such low-mass
galaxies are of considerable interest as the low-redshift analogs to the seeds
of the most massive BHs at high redshift which have remained largely elusive to
date. Our study highlights the power of deep X-ray surveys to uncover such
low-mass systems.
We report the discovery of three low-mass black hole candidates residing in
the centers of low-mass galaxies at z<0.3 in the Chandra Deep Field - South
Survey. These black holes are initially identified as candidate active galactic
nuclei based on their X-ray emission in deep Chandra observations.
Multi-wavelength observations are used to strengthen our claim that such
emission is powered by an accreting supermassive black hole. While the X-ray
luminosities are low at L_X ~ 10^40 erg s^-1 (and variable in one case), we
argue that they are unlikely to be attributed to star formation based on
H\alpha or UV-fluxes. Optical spectroscopy with Keck/DEIMOS and VLT/FORS allows
us to (1) measure accurate redshifts, (2) confirm their low stellar host mass,
(3) investigate the source(s) of photo-ionization, and (4) estimate extinction.
With stellar masses of M* < 3*10^9 M_\sun determined from HST/ACS imaging, the
host galaxies are among the lowest mass systems known to host actively
accreting black holes. We estimate BH masses M_BH ~ 2*10^5 M_\sun based on
scaling relations between BH mass and host properties for more luminous
systems. In one case, a broad component of the H\alpha emission-line profile is
detected thus providing a virial mass estimate. Black holes in such low-mass
galaxies are of considerable interest as the low-redshift analogs to the seeds
of the most massive BHs at high redshift which have remained largely elusive to
date. Our study highlights the power of deep X-ray surveys to uncover such
low-mass systems.
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