Validity of Hydrostatic Equilibrium in Galaxy Clusters from Cosmological Hydrodynamical Simulations. (arXiv:1302.5172v1 [astro-ph.CO]):
We examine the validity of the hydrostatic equilibrium (HSE) assumption for
galaxy clusters using one of the highest-resolution cosmological hydrodynamical
simulations. We define and evaluate several effective mass terms corresponding
to the Euler equations of the gas dynamics, and quantify the degree of the
validity of HSE in terms of the mass estimate. We find that the mass estimated
under the HSE assumption (the HSE mass) deviates from the true mass by up to ~
30 %. This level of departure from HSE is consistent with the previous claims,
but our physical interpretation is rather different. We demonstrate that the
inertial term in the Euler equations makes a negligible contribution to the
total mass, and the overall gravity of the cluster is balanced by the thermal
gas pressure gradient and the gas acceleration term. Indeed the deviation from
the HSE mass is well explained by the acceleration term at almost all radii. We
also clarify the confusion of previous work due to the inappropriate
application of the Jeans equations in considering the validity of HSE from the
gas dynamics extracted from cosmological hydrodynamical simulations.
Showing posts with label Theory. Show all posts
Showing posts with label Theory. Show all posts
Saturday, March 9, 2013
Sunday, March 3, 2013
Conditions For Successful Helium Detonations In Astrophysical Environments. (arXiv:1302.6235v1 [astro-ph.HE])
Conditions For Successful Helium Detonations In Astrophysical Environments. (arXiv:1302.6235v1 [astro-ph.HE]):
Several models for type Ia-like supernovae events rely on the production of a
self-sustained detonation powered by nuclear reactions.In the absence of
hydrogen, the fuel that powers these detonations typically consists of either
pure helium (He) or a mixture of carbon and oxygen (C/O). Studies that
systematically determine the conditions required to initiate detonations in C/O
material exist, but until now no analogous investigation of degenerate He
matter has been conducted. We perform one-dimensional reactive hydrodynamical
simulations at a variety of initial density and temperature combinations and
find critical length scales for the initiation of He detonations that range
between 1 -- $10^{10}$ cm. These sizes are consistently smaller than the
corresponding Chapman-Jouguet (CJ) length scales by a factor of ~100, providing
opportunities for thermonuclear explosions in a wider range of low mass white
dwarfs (WDs) than previously thought possible. We find that virialized WDs with
as little mass as 0.24 $M_\odot$ can be detonated, and that even less massive
WDs can be detonated if a sizable fraction of their mass is raised to a higher
adiabat. That the initiation length is exceeded by the CJ length implies that
certain systems may not reach nuclear statistical equilibrium within the time
it takes a detonation to traverse the object. In support of this hypothesis, we
demonstrate that incomplete burning will occur in the majority of He WD
detonations and that $^{44}$Ti, rather than $^{56}$Ni, is the predominant
burning product for many of these events. We anticipate that a measure of the
quantity of $^{44}$Ti and $^{56}$Ni produced in a helium-rich thermonuclear
explosion can potentially be used to constrain the nature of the progenitor
system.
Several models for type Ia-like supernovae events rely on the production of a
self-sustained detonation powered by nuclear reactions.In the absence of
hydrogen, the fuel that powers these detonations typically consists of either
pure helium (He) or a mixture of carbon and oxygen (C/O). Studies that
systematically determine the conditions required to initiate detonations in C/O
material exist, but until now no analogous investigation of degenerate He
matter has been conducted. We perform one-dimensional reactive hydrodynamical
simulations at a variety of initial density and temperature combinations and
find critical length scales for the initiation of He detonations that range
between 1 -- $10^{10}$ cm. These sizes are consistently smaller than the
corresponding Chapman-Jouguet (CJ) length scales by a factor of ~100, providing
opportunities for thermonuclear explosions in a wider range of low mass white
dwarfs (WDs) than previously thought possible. We find that virialized WDs with
as little mass as 0.24 $M_\odot$ can be detonated, and that even less massive
WDs can be detonated if a sizable fraction of their mass is raised to a higher
adiabat. That the initiation length is exceeded by the CJ length implies that
certain systems may not reach nuclear statistical equilibrium within the time
it takes a detonation to traverse the object. In support of this hypothesis, we
demonstrate that incomplete burning will occur in the majority of He WD
detonations and that $^{44}$Ti, rather than $^{56}$Ni, is the predominant
burning product for many of these events. We anticipate that a measure of the
quantity of $^{44}$Ti and $^{56}$Ni produced in a helium-rich thermonuclear
explosion can potentially be used to constrain the nature of the progenitor
system.
Sunday, February 17, 2013
X-ray C-M relation: theory and observation. (arXiv:1301.7476v1 [astro-ph.CO])
X-ray C-M relation: theory and observation. (arXiv:1301.7476v1 [astro-ph.CO]):
[Abridged] Since fifteen years, the concentration-mass relation has been
investigated diffusely in theoretical studies. On the other hand, only recently
this relation has been derived from X-ray observations. When that happened, the
results caused a certain level of concern: the X-ray normalizations and slopes
were found significantly dissimilar from those predicted by theory.
By analyzing a total of 52 objects, simulated each time with different
physical recipes for the baryonic component, as well as 60 synthetic X-ray
images, we aim at determining if these discrepancies are real or artificial. In
particular, we investigate how the simulated concentration-mass relation
depends i) on the radial range used to derive the concentration, ii) on the
presence of baryons in the simulations, and on the prescription used to
reproduce the gas. Finally, we evaluate iii) how the results differ when
adopting an X-ray approach for the analysis and iv) how the selection functions
based on X-ray luminosity, temperature, and SZ-signal can impact the results.
All effects studied go in the direction of explaining the discrepancy between
observations and simulations, but they contribute at different levels: while
the fitting radial range and the baryonic component play only a minor role, the
X-ray approach and selection function have profound repercussion on the
results.
RKS Note: Important paper comparing simulated clusters with observed ones, especially in regard to extracting cosmological parameters.
[Abridged] Since fifteen years, the concentration-mass relation has been
investigated diffusely in theoretical studies. On the other hand, only recently
this relation has been derived from X-ray observations. When that happened, the
results caused a certain level of concern: the X-ray normalizations and slopes
were found significantly dissimilar from those predicted by theory.
By analyzing a total of 52 objects, simulated each time with different
physical recipes for the baryonic component, as well as 60 synthetic X-ray
images, we aim at determining if these discrepancies are real or artificial. In
particular, we investigate how the simulated concentration-mass relation
depends i) on the radial range used to derive the concentration, ii) on the
presence of baryons in the simulations, and on the prescription used to
reproduce the gas. Finally, we evaluate iii) how the results differ when
adopting an X-ray approach for the analysis and iv) how the selection functions
based on X-ray luminosity, temperature, and SZ-signal can impact the results.
All effects studied go in the direction of explaining the discrepancy between
observations and simulations, but they contribute at different levels: while
the fitting radial range and the baryonic component play only a minor role, the
X-ray approach and selection function have profound repercussion on the
results.
RKS Note: Important paper comparing simulated clusters with observed ones, especially in regard to extracting cosmological parameters.
Wednesday, January 23, 2013
Irradiation of an Accretion Disc by a Jet: General Properties and Implications for Spin Measurements of Black Holes. (arXiv:1301.4922v1 [astro-ph.HE])
Irradiation of an Accretion Disc by a Jet: General Properties and Implications for Spin Measurements of Black Holes. (arXiv:1301.4922v1 [astro-ph.HE]):
X-ray irradiation of the accretion disc leads to strong reflection features,
which are then broadened and distorted by relativistic effects. We present a
detailed, general relativistic approach to model this irradiation for different
geometries of the primary X-ray source. These geometries include the standard
point source on the rotational axis as well as more jet-like sources, which are
radially elongated and accelerating. Incorporating this code in the relline
model for relativistic line emission, the line shape for any configuration can
be predicted. We study how different irradiation geometries affect the
determination of the spin of the black hole. Broad emission lines are produced
only for compact irradiating sources situated close to the black hole. This is
the only case where the black hole spin can be unambiguously determined. In all
other cases the line shape is narrower, which could either be explained by a
low spin or an elongated source. We conclude that for all those cases and
independent of the quality of the data, no unique solution for the spin value
exists.
X-ray irradiation of the accretion disc leads to strong reflection features,
which are then broadened and distorted by relativistic effects. We present a
detailed, general relativistic approach to model this irradiation for different
geometries of the primary X-ray source. These geometries include the standard
point source on the rotational axis as well as more jet-like sources, which are
radially elongated and accelerating. Incorporating this code in the relline
model for relativistic line emission, the line shape for any configuration can
be predicted. We study how different irradiation geometries affect the
determination of the spin of the black hole. Broad emission lines are produced
only for compact irradiating sources situated close to the black hole. This is
the only case where the black hole spin can be unambiguously determined. In all
other cases the line shape is narrower, which could either be explained by a
low spin or an elongated source. We conclude that for all those cases and
independent of the quality of the data, no unique solution for the spin value
exists.
Evolution of M82-like starburst winds revisited: 3D radiative cooling hydrodynamical simulations. (arXiv:1301.5005v1 [astro-ph.CO])
Evolution of M82-like starburst winds revisited: 3D radiative cooling hydrodynamical simulations. (arXiv:1301.5005v1 [astro-ph.CO]):
In this study we present three-dimensional radiative cooling hydrodynamical
simulations of galactic winds generated particularly in M82-like starburst
galaxies. We have considered intermittent winds induced by SNe explosions
within super star clusters randomly distributed in the central region of the
galaxy and were able to reproduce the observed M82 wind conditions with its
complex morphological outflow structure. We have found that the environmental
conditions in the disk in nearly recent past are crucial to determine whether
the wind will develop a large scale rich filamentary structure, as in M82 wind,
or not. Also, the numerical evolution of the SN ejecta have allowed us to
obtain the abundance distribution over the first 3 kpc extension of the wind
and we have found that the SNe explosions change significantly the metallicity
only of the hot, low-density wind component. Moreover, we have found that the
SN-driven wind transports to outside the disk large amounts of energy, momentum
and gas, but the more massive high-density component reaches only intermediate
altitudes smaller than 1.5 kpc. Therefore, no significant amounts of gas mass
are lost to the IGM and the mass evolution of the galaxy is not much affected
by the starburst events occurring in the nuclear region.
In this study we present three-dimensional radiative cooling hydrodynamical
simulations of galactic winds generated particularly in M82-like starburst
galaxies. We have considered intermittent winds induced by SNe explosions
within super star clusters randomly distributed in the central region of the
galaxy and were able to reproduce the observed M82 wind conditions with its
complex morphological outflow structure. We have found that the environmental
conditions in the disk in nearly recent past are crucial to determine whether
the wind will develop a large scale rich filamentary structure, as in M82 wind,
or not. Also, the numerical evolution of the SN ejecta have allowed us to
obtain the abundance distribution over the first 3 kpc extension of the wind
and we have found that the SNe explosions change significantly the metallicity
only of the hot, low-density wind component. Moreover, we have found that the
SN-driven wind transports to outside the disk large amounts of energy, momentum
and gas, but the more massive high-density component reaches only intermediate
altitudes smaller than 1.5 kpc. Therefore, no significant amounts of gas mass
are lost to the IGM and the mass evolution of the galaxy is not much affected
by the starburst events occurring in the nuclear region.
Sunday, January 20, 2013
Perspectives on Intracluster Enrichment and the Stellar Initial Mass Function in Elliptical Galaxies. (arXiv:1301.3200v1 [astro-ph.CO])
Perspectives on Intracluster Enrichment and the Stellar Initial Mass Function in Elliptical Galaxies. (arXiv:1301.3200v1 [astro-ph.CO]):
The amount of metals in the Intracluster Medium (ICM) in rich galaxy clusters
exceeds that expected based on the observed stellar population by a large
factor. We quantify this discrepancy -- which we term the "cluster elemental
abundance paradox" -- and investigate the required properties of the
ICM-enriching population. The necessary enhancement in metal enrichment may, in
principle, originate in the observed stellar population if a larger fraction of
stars in the supernova-progenitor mass range form from an initial mass function
(IMF) that is either bottom-light or top-heavy, with the latter in some
conflict with observed ICM abundance ratios. Other alternatives that imply more
modest revisions to the IMF, mass return and remnant fractions, and primordial
fraction, posit an increase in the fraction of 3-8 solar mass stars that
explode as SNIa or assume that there are more stars than conventionally thought
-- although the latter implies a high star formation efficiency. We discuss the
feasibility of these various solutions and the implications for the diversity
of star formation, the process of elliptical galaxy formation, and the nature
of this "hidden" source of ICM metal enrichment in light of recent evidence of
an elliptical galaxy IMF that, because it is skewed to low masses, deepens the
paradox.
The amount of metals in the Intracluster Medium (ICM) in rich galaxy clusters
exceeds that expected based on the observed stellar population by a large
factor. We quantify this discrepancy -- which we term the "cluster elemental
abundance paradox" -- and investigate the required properties of the
ICM-enriching population. The necessary enhancement in metal enrichment may, in
principle, originate in the observed stellar population if a larger fraction of
stars in the supernova-progenitor mass range form from an initial mass function
(IMF) that is either bottom-light or top-heavy, with the latter in some
conflict with observed ICM abundance ratios. Other alternatives that imply more
modest revisions to the IMF, mass return and remnant fractions, and primordial
fraction, posit an increase in the fraction of 3-8 solar mass stars that
explode as SNIa or assume that there are more stars than conventionally thought
-- although the latter implies a high star formation efficiency. We discuss the
feasibility of these various solutions and the implications for the diversity
of star formation, the process of elliptical galaxy formation, and the nature
of this "hidden" source of ICM metal enrichment in light of recent evidence of
an elliptical galaxy IMF that, because it is skewed to low masses, deepens the
paradox.
Wednesday, December 19, 2012
The Escape Fraction of Ionizing Radiation from Primordial Galaxies. (arXiv:1212.4452v1 [astro-ph.CO])
The Escape Fraction of Ionizing Radiation from Primordial Galaxies. (arXiv:1212.4452v1 [astro-ph.CO]):
The escape of ionizing radiation from galaxies plays a critical role in the
evolution of gas in galaxies, and the heating and ionization history of the
intergalactic medium. Here, we present semi-analytic calculations of the escape
fraction of ionizing radiation for both hydrogen and helium from primordial
galaxies, as well as analytic derivations of these quantities. We consider
variations in the galaxy density profile, source type, location, and spectrum,
and gas clumping/distribution factors. For sufficiently hard first-light
sources, the helium ionization fronts closely track or even advance beyond that
of hydrogen. Key new results in this work include calculations of the escape
fractions for He I and He II ionizing radiation, and the impact of partial
ionization from X-rays from early AGN or stellar clusters on the escape
fractions from primordial halos. When factoring in frequency-dependent effects,
we find that X-rays play an important role in boosting the escape fractions for
both hydrogen and helium, but especially for He II. We briefly discuss the
implications of these results for recent observations of the He II reionization
epoch at low redshifts, as well as the UV data and emission-line signatures
from early galaxies anticipated from future satellite missions.
The escape of ionizing radiation from galaxies plays a critical role in the
evolution of gas in galaxies, and the heating and ionization history of the
intergalactic medium. Here, we present semi-analytic calculations of the escape
fraction of ionizing radiation for both hydrogen and helium from primordial
galaxies, as well as analytic derivations of these quantities. We consider
variations in the galaxy density profile, source type, location, and spectrum,
and gas clumping/distribution factors. For sufficiently hard first-light
sources, the helium ionization fronts closely track or even advance beyond that
of hydrogen. Key new results in this work include calculations of the escape
fractions for He I and He II ionizing radiation, and the impact of partial
ionization from X-rays from early AGN or stellar clusters on the escape
fractions from primordial halos. When factoring in frequency-dependent effects,
we find that X-rays play an important role in boosting the escape fractions for
both hydrogen and helium, but especially for He II. We briefly discuss the
implications of these results for recent observations of the He II reionization
epoch at low redshifts, as well as the UV data and emission-line signatures
from early galaxies anticipated from future satellite missions.
Monday, November 12, 2012
Disentangling Resonant Scattering and Gas Motions in Galaxy Cluster Emission Line Profiles. (arXiv:1211.2375v1 [astro-ph.CO])
Disentangling Resonant Scattering and Gas Motions in Galaxy Cluster Emission Line Profiles. (arXiv:1211.2375v1 [astro-ph.CO]):
Future high spectral resolution telescopes will enable us to place tight
constraints on turbulence in the intra-cluster medium through the line widths
of strong emission lines. At the same time, these bright lines are the most
prone to be optically thick. This requires us to separate the effects of
resonant scattering from turbulence, both of which could broaden the lines. How
this can be achieved has yet not been quantitatively addressed. In this paper,
we propose a flexible new parametrization for the line profile, which allows
these effects to be distinguished. The model has only 3 free parameters, which
we calibrate with Monte-Carlo radiative transfer simulations. We provide
fitting functions and tables that allow the results of these calculations to be
easily incorporated into a fast spectral fitting package. In a mock spectral
fit, we explicitly show that this parameterization allows us to correctly
estimate the turbulent amplitude and metallicity of a cluster such as Perseus,
which would otherwise give significantly biased results. We also show how the
physical origin of the line shape can be understood analytically.
Future high spectral resolution telescopes will enable us to place tight
constraints on turbulence in the intra-cluster medium through the line widths
of strong emission lines. At the same time, these bright lines are the most
prone to be optically thick. This requires us to separate the effects of
resonant scattering from turbulence, both of which could broaden the lines. How
this can be achieved has yet not been quantitatively addressed. In this paper,
we propose a flexible new parametrization for the line profile, which allows
these effects to be distinguished. The model has only 3 free parameters, which
we calibrate with Monte-Carlo radiative transfer simulations. We provide
fitting functions and tables that allow the results of these calculations to be
easily incorporated into a fast spectral fitting package. In a mock spectral
fit, we explicitly show that this parameterization allows us to correctly
estimate the turbulent amplitude and metallicity of a cluster such as Perseus,
which would otherwise give significantly biased results. We also show how the
physical origin of the line shape can be understood analytically.
Friday, November 9, 2012
Three-dimensional numerical investigations of the morphology of type Ia SNRs. (arXiv:1210.7790v1 [astro-ph.GA])
Three-dimensional numerical investigations of the morphology of type Ia SNRs. (arXiv:1210.7790v1 [astro-ph.GA]):
We explore the morphology of Type Ia supernova remnants (SNRs) using
three-dimensional hydrodynamics modeling and an exponential density profile.
Our model distinguishes ejecta from the interstellar medium (ISM), and tracks
the ionization age of shocked ejecta, both of which allow for additional
analysis of the simulated remnants. We also include the adiabatic index as a
free parameter, which affects the compressibility of the fluid and emulates the
efficiency of cosmic ray acceleration by shock fronts. In addition to
generating 3-D images of the simulations, we compute line-of-sight projections
through the remnants for comparison against observations of Tycho's SNR and SN
1006. We find that several features observed in these two remnants, such as the
separation between the fluid discontinuities and the presence of ejecta knots
ahead of the forward shock, can be generated by smooth ejecta without any
initial clumpiness. Our results are consistent with SN 1006 being dynamically
younger than Tycho's SNR, and more efficiently accelerating cosmic rays at its
forward shock. We conclude that clumpiness is not a necessary condition to
reproduce many observed features of Type Ia supernova remnants, particularly
the radial profiles and the fleecy emission from ejecta at the central region
of both remnants.
We explore the morphology of Type Ia supernova remnants (SNRs) using
three-dimensional hydrodynamics modeling and an exponential density profile.
Our model distinguishes ejecta from the interstellar medium (ISM), and tracks
the ionization age of shocked ejecta, both of which allow for additional
analysis of the simulated remnants. We also include the adiabatic index as a
free parameter, which affects the compressibility of the fluid and emulates the
efficiency of cosmic ray acceleration by shock fronts. In addition to
generating 3-D images of the simulations, we compute line-of-sight projections
through the remnants for comparison against observations of Tycho's SNR and SN
1006. We find that several features observed in these two remnants, such as the
separation between the fluid discontinuities and the presence of ejecta knots
ahead of the forward shock, can be generated by smooth ejecta without any
initial clumpiness. Our results are consistent with SN 1006 being dynamically
younger than Tycho's SNR, and more efficiently accelerating cosmic rays at its
forward shock. We conclude that clumpiness is not a necessary condition to
reproduce many observed features of Type Ia supernova remnants, particularly
the radial profiles and the fleecy emission from ejecta at the central region
of both remnants.
Thursday, October 25, 2012
Theoretical Studies of Accretion of Matter onto White Dwarfs and the Single Degenerate Scenario for Supernovae of Type Ia. (arXiv:1210.6086v1 [astro-ph.SR])
Theoretical Studies of Accretion of Matter onto White Dwarfs and the Single Degenerate Scenario for Supernovae of Type Ia. (arXiv:1210.6086v1 [astro-ph.SR]):
We present a brief summary of the Single Degenerate Scenario for the
progenitors of Type Ia Supernovae in which it is assumed that a low mass
carbon-oxygen white dwarf is growing in mass as a result of accretion from a
secondary star in a close binary system. Recent hydrodynamic simulations of
accretion of solar material onto white dwarfs without mixing always produce a
thermonuclear runaway and steady burning does not occur. For a broad range in
WD mass (0.4 Solar masses to 1.35 Solar Masses), the maximum ejected material
occurs for the 1.25 Solar Mass sequences and then decreases as the white dwarf
mass decreases. Therefore, the white dwarfs are growing in mass as a
consequence of the accretion of solar material and as long as there is no
mixing of accreted material with core material. In contrast, a thermonuclear
runaway in the accreted hydrogen-rich layers on the low luminosity WDs in close
binary systems where mixing of core matter with accreted material has occurred
is the outburst mechanism for Classical, Recurrent, and Symbiotic novae. The
differences in characteristics of these systems is likely the WD mass and mass
accretion rate. The high levels of enrichment of CN ejecta in elements ranging
from carbon to sulfur confirm that there is dredge-up of matter from the core
of the WD and enable them to contribute to the chemical enrichment of the
interstellar medium. Therefore, studies of CNe can lead to an improved
understanding of Galactic nucleosynthesis, some sources of pre-solar grains,
and the Extragalactic distance scale. The characteristics of the outburst
depend on the white dwarf mass, luminosity, mass accretion rate, and the
chemical composition of both the accreting material and WD material. The
properties of the outburst also depends on when, how, and if the accreted
layers are mixed with the WD core and the mixing mechanism is still unknown.
We present a brief summary of the Single Degenerate Scenario for the
progenitors of Type Ia Supernovae in which it is assumed that a low mass
carbon-oxygen white dwarf is growing in mass as a result of accretion from a
secondary star in a close binary system. Recent hydrodynamic simulations of
accretion of solar material onto white dwarfs without mixing always produce a
thermonuclear runaway and steady burning does not occur. For a broad range in
WD mass (0.4 Solar masses to 1.35 Solar Masses), the maximum ejected material
occurs for the 1.25 Solar Mass sequences and then decreases as the white dwarf
mass decreases. Therefore, the white dwarfs are growing in mass as a
consequence of the accretion of solar material and as long as there is no
mixing of accreted material with core material. In contrast, a thermonuclear
runaway in the accreted hydrogen-rich layers on the low luminosity WDs in close
binary systems where mixing of core matter with accreted material has occurred
is the outburst mechanism for Classical, Recurrent, and Symbiotic novae. The
differences in characteristics of these systems is likely the WD mass and mass
accretion rate. The high levels of enrichment of CN ejecta in elements ranging
from carbon to sulfur confirm that there is dredge-up of matter from the core
of the WD and enable them to contribute to the chemical enrichment of the
interstellar medium. Therefore, studies of CNe can lead to an improved
understanding of Galactic nucleosynthesis, some sources of pre-solar grains,
and the Extragalactic distance scale. The characteristics of the outburst
depend on the white dwarf mass, luminosity, mass accretion rate, and the
chemical composition of both the accreting material and WD material. The
properties of the outburst also depends on when, how, and if the accreted
layers are mixed with the WD core and the mixing mechanism is still unknown.
Monday, October 15, 2012
Thermal and non-thermal traces of AGN feedback: results from cosmological AMR simulations. (arXiv:1210.3541v1 [astro-ph.CO])
Thermal and non-thermal traces of AGN feedback: results from cosmological AMR simulations. (arXiv:1210.3541v1 [astro-ph.CO]):
We investigate the observable effects of feedback from Active Galactic Nuclei
(AGN) on non-thermal components of the intracluster medium (ICM). We have
modelled feedback from AGN in cosmological simulations with the adaptive mesh
refinement code ENZO, investigating three types of feedback that are sometimes
called quasar, jet and radio mode. Using a small set of galaxy clusters
simulated at high resolution, we model the injection and evolution of Cosmic
Rays, as well as their effects on the thermal plasma. By comparing, both, the
profiles of thermal gas to observed profiles from the ACCEPT sample, and the
secondary gamma-ray emission to the available upper limits from FERMI, we
discuss how the combined analysis of these two observables can constrain the
energetics and mechanisms of feedback models in clusters. Those modes of AGN
feedback that provide a good match to X-ray observations, yield a gamma-ray
luminosity resulting from secondary cosmic rays that is about below the
available upper limits from FERMI. Moreover, we investigate the injection of
turbulent motions into the ICM from AGN, and the detectability of these motions
via the analysis of line broadening of the Fe XXIII line. In the near future,
deeper observations/upper-limits of non-thermal emissions from galaxy clusters
will yield stringent constraints on the energetics and modes of AGN feedback,
even at early cosmic epochs.
We investigate the observable effects of feedback from Active Galactic Nuclei
(AGN) on non-thermal components of the intracluster medium (ICM). We have
modelled feedback from AGN in cosmological simulations with the adaptive mesh
refinement code ENZO, investigating three types of feedback that are sometimes
called quasar, jet and radio mode. Using a small set of galaxy clusters
simulated at high resolution, we model the injection and evolution of Cosmic
Rays, as well as their effects on the thermal plasma. By comparing, both, the
profiles of thermal gas to observed profiles from the ACCEPT sample, and the
secondary gamma-ray emission to the available upper limits from FERMI, we
discuss how the combined analysis of these two observables can constrain the
energetics and mechanisms of feedback models in clusters. Those modes of AGN
feedback that provide a good match to X-ray observations, yield a gamma-ray
luminosity resulting from secondary cosmic rays that is about below the
available upper limits from FERMI. Moreover, we investigate the injection of
turbulent motions into the ICM from AGN, and the detectability of these motions
via the analysis of line broadening of the Fe XXIII line. In the near future,
deeper observations/upper-limits of non-thermal emissions from galaxy clusters
will yield stringent constraints on the energetics and modes of AGN feedback,
even at early cosmic epochs.
Friday, October 5, 2012
A Metric for Testing the Nature of Black Holes. (arXiv:1210.0483v1 [gr-qc])
A Metric for Testing the Nature of Black Holes. (arXiv:1210.0483v1 [gr-qc]):
In general relativity, astrophysical black holes are uniquely described by
the Kerr metric. Observational tests of the Kerr nature of these compact
objects and, hence, of general relativity, require a metric that encompasses a
broader class of black holes as possible alternatives to the usual Kerr black
holes. Several such Kerr-like metrics have been constructed to date, which
depend on a set of free parameters and which reduce smoothly to the Kerr metric
if all deviations vanish. Many of these metrics, however, are valid only for
small values of the spin or small perturbations of the Kerr metric or contain
regions of space where they are unphysical hampering their ability to properly
model the accretions flows of black holes. In this paper, I describe a
Kerr-like black hole metric that is regular everywhere outside of the event
horizon for black holes with arbitrary spins even for large deviations from the
Kerr metric. This metric, therefore, provides an ideal framework for tests of
the nature of black holes with observations of the emission from their
accretion flows, and I give several examples of such tests across the
electromagnetic spectrum with current and near-future instruments.
Note: This paper notes how broadened Fe-K lines could test the black hole 'No-Hair' Theorem, with predictions of line shapes.
In general relativity, astrophysical black holes are uniquely described by
the Kerr metric. Observational tests of the Kerr nature of these compact
objects and, hence, of general relativity, require a metric that encompasses a
broader class of black holes as possible alternatives to the usual Kerr black
holes. Several such Kerr-like metrics have been constructed to date, which
depend on a set of free parameters and which reduce smoothly to the Kerr metric
if all deviations vanish. Many of these metrics, however, are valid only for
small values of the spin or small perturbations of the Kerr metric or contain
regions of space where they are unphysical hampering their ability to properly
model the accretions flows of black holes. In this paper, I describe a
Kerr-like black hole metric that is regular everywhere outside of the event
horizon for black holes with arbitrary spins even for large deviations from the
Kerr metric. This metric, therefore, provides an ideal framework for tests of
the nature of black holes with observations of the emission from their
accretion flows, and I give several examples of such tests across the
electromagnetic spectrum with current and near-future instruments.
Note: This paper notes how broadened Fe-K lines could test the black hole 'No-Hair' Theorem, with predictions of line shapes.
Tuesday, September 18, 2012
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.
Tuesday, September 4, 2012
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.
Saturday, September 1, 2012
On the diversity and complexity of absorption line profiles produced by outflows in Active Galactic Nuclei. (arXiv:1208.6044v1 [astro-ph.CO])
On the diversity and complexity of absorption line profiles produced by outflows in Active Galactic Nuclei. (arXiv:1208.6044v1 [astro-ph.CO]):
Understanding the origin of AGN absorption line profiles and their diversity
could help to explain the physical structure of the accretion flow, and also to
assess the impact of accretion on the evolution of the AGN host galaxies. Here
we present our first attempt to systematically address the issue of the origin
of the complexities observed in absorption profiles. Using a simple method, we
compute absorption line profiles against a continuum point source for several
simulations of accretion disk winds. We investigate the geometrical,
ionization, and dynamical effects on the absorption line shapes. We find that
significant complexity and diversity of the absorption line profile shapes can
be produced by the non-monotonic distribution of the wind velocity, density,
and ionization state. Non-monotonic distributions of such quantities are
present even in steady-state, smooth disk winds, and naturally lead to the
formation of multiple and detached absorption troughs. These results
demonstrate that the part of a wind where an absorption line is formed is not
representative of the entire wind. Thus, the information contained in the
absorption line is incomplete if not even insufficient to well estimate gross
properties of the wind such as the total mass and energy fluxes. In addition,
the highly dynamical nature of certain portions of disk winds can have
important effects on the estimates of the wind properties. For example, the
mass outflow rates can be off up to two orders of magnitude with respect to
estimates based on a spherically symmetric, homogeneous, constant velocity
wind.
Understanding the origin of AGN absorption line profiles and their diversity
could help to explain the physical structure of the accretion flow, and also to
assess the impact of accretion on the evolution of the AGN host galaxies. Here
we present our first attempt to systematically address the issue of the origin
of the complexities observed in absorption profiles. Using a simple method, we
compute absorption line profiles against a continuum point source for several
simulations of accretion disk winds. We investigate the geometrical,
ionization, and dynamical effects on the absorption line shapes. We find that
significant complexity and diversity of the absorption line profile shapes can
be produced by the non-monotonic distribution of the wind velocity, density,
and ionization state. Non-monotonic distributions of such quantities are
present even in steady-state, smooth disk winds, and naturally lead to the
formation of multiple and detached absorption troughs. These results
demonstrate that the part of a wind where an absorption line is formed is not
representative of the entire wind. Thus, the information contained in the
absorption line is incomplete if not even insufficient to well estimate gross
properties of the wind such as the total mass and energy fluxes. In addition,
the highly dynamical nature of certain portions of disk winds can have
important effects on the estimates of the wind properties. For example, the
mass outflow rates can be off up to two orders of magnitude with respect to
estimates based on a spherically symmetric, homogeneous, constant velocity
wind.
Thursday, August 30, 2012
How Baryonic Processes affect Strong Lensing properties of Simulated Galaxy Clusters. (arXiv:1208.5770v1 [astro-ph.CO])
How Baryonic Processes affect Strong Lensing properties of Simulated Galaxy Clusters. (arXiv:1208.5770v1 [astro-ph.CO]):
The observed abundance of giant arcs produced by galaxy cluster lenses and
the measured Einstein radii have presented a source of tension for LCDM.
Previous cosmological tests for high-redshift clusters (z>0.5) have suffered
from small number statistics in the simulated sample and the implementation of
baryonic physics is likely to affect the outcome. We analyse zoomed-in
simulations of a fairly large sample of cluster-sized objects, with Mvir >
3x10^14 Msun/h, identified at z=0.25 and z=0.5, for a concordance LCDM
cosmology. We start with dark matter only simulations, and then add gas
hydrodynamics, with different treatments of baryonic processes: non-radiative
cooling, radiative cooling with star formation and galactic winds powered by
supernova explosions, and finally including the effect of AGN feedback. We find
that the addition of gas in non-radiative simulations does not change the
strong lensing predictions significantly, but gas cooling and star formation
together significantly increase the number of expected giant arcs and the
Einstein radii, particularly for lower redshift clusters and lower source
redshifts. Further inclusion of AGN feedback reduces the predicted strong
lensing efficiencies such that the lensing probability distributions becomes
closer to those obtained for simulations including only dark matter. Our
results indicate that the inclusion of baryonic physics in simulations will not
solve the arc-statistics problem at low redshifts, when the physical processes
included provide a realistic description of cooling in the central regions of
galaxy clusters. [Abridged]
The observed abundance of giant arcs produced by galaxy cluster lenses and
the measured Einstein radii have presented a source of tension for LCDM.
Previous cosmological tests for high-redshift clusters (z>0.5) have suffered
from small number statistics in the simulated sample and the implementation of
baryonic physics is likely to affect the outcome. We analyse zoomed-in
simulations of a fairly large sample of cluster-sized objects, with Mvir >
3x10^14 Msun/h, identified at z=0.25 and z=0.5, for a concordance LCDM
cosmology. We start with dark matter only simulations, and then add gas
hydrodynamics, with different treatments of baryonic processes: non-radiative
cooling, radiative cooling with star formation and galactic winds powered by
supernova explosions, and finally including the effect of AGN feedback. We find
that the addition of gas in non-radiative simulations does not change the
strong lensing predictions significantly, but gas cooling and star formation
together significantly increase the number of expected giant arcs and the
Einstein radii, particularly for lower redshift clusters and lower source
redshifts. Further inclusion of AGN feedback reduces the predicted strong
lensing efficiencies such that the lensing probability distributions becomes
closer to those obtained for simulations including only dark matter. Our
results indicate that the inclusion of baryonic physics in simulations will not
solve the arc-statistics problem at low redshifts, when the physical processes
included provide a realistic description of cooling in the central regions of
galaxy clusters. [Abridged]
Sunday, August 12, 2012
X-ray photoionized bubble in the wind of Vela X-1 pulsar supergiant companion. (arXiv:1208.1827v1 [astro-ph.SR])
X-ray photoionized bubble in the wind of Vela X-1 pulsar supergiant companion. (arXiv:1208.1827v1 [astro-ph.SR]):
Vela X-1 is the archetype of high-mass X-ray binaries, composed of a neutron
star and a massive B supergiant. The supergiant is a source of a strong
radiatively-driven stellar wind. The neutron star sweeps up this wind, and
creates a huge amount of X-rays as a result of energy release during the
process of wind accretion. Here we provide detailed NLTE models of the Vela X-1
envelope. We study how the X-rays photoionize the wind and destroy the ions
responsible for the wind acceleration. The resulting decrease of the radiative
force explains the observed reduction of the wind terminal velocity in a
direction to the neutron star. The X-rays create a distinct photoionized region
around the neutron star filled with a stagnating flow. The existence of such
photoionized bubbles is a general property of high-mass X-ray binaries. We
unveiled a new principle governing these complex objects, according to which
there is an upper limit to the X-ray luminosity the compact star can have
without suspending the wind due to inefficient line driving
Vela X-1 is the archetype of high-mass X-ray binaries, composed of a neutron
star and a massive B supergiant. The supergiant is a source of a strong
radiatively-driven stellar wind. The neutron star sweeps up this wind, and
creates a huge amount of X-rays as a result of energy release during the
process of wind accretion. Here we provide detailed NLTE models of the Vela X-1
envelope. We study how the X-rays photoionize the wind and destroy the ions
responsible for the wind acceleration. The resulting decrease of the radiative
force explains the observed reduction of the wind terminal velocity in a
direction to the neutron star. The X-rays create a distinct photoionized region
around the neutron star filled with a stagnating flow. The existence of such
photoionized bubbles is a general property of high-mass X-ray binaries. We
unveiled a new principle governing these complex objects, according to which
there is an upper limit to the X-ray luminosity the compact star can have
without suspending the wind due to inefficient line driving
Wednesday, August 1, 2012
Synthetic X-ray spectra for simulations of the dynamics of an accretion flow irradiated by a quasar. (arXiv:1207.7194v1 [astro-ph.HE])
Synthetic X-ray spectra for simulations of the dynamics of an accretion flow irradiated by a quasar. (arXiv:1207.7194v1 [astro-ph.HE]):
Ultraviolet and X-ray observations show evidence of outflowing gas around
many active galactic nuclei. Some of these outflows may be driven off gas
infalling towards the central black hole. We perform radiative transfer
calculations to compute the gas ionization state and X-ray spectra for two- and
three-dimensional (3D) hydrodynamical simulations of this outflow-from-inflow
scenario. By comparison with observations, our results can be used to test the
theoretical models and guide future numerical simulations. We predict both
absorption and emission features, most of which are formed in a polar funnel of
outflowing gas. This outflow causes strong absorption for observer orientation
angles of < 35 degrees. Particularly in 3D, the strength of this absorption
varies significantly for different lines-of-sight owing to the fragmentary
structure of the gas flow. Although infalling material occupies a large
fraction of the simulation volume, we do not find that it imprints strong
absorption features since the ionization state is very high. Thus, an absence
of observed inflow absorption features does not exclude the models. The main
spectroscopic consequence of the infalling gas is a scattered continuum
component that partially re-fills the absorption features caused by the
outflowing polar funnel. Fluorescence and scattering in the outflow is
predicted to give rise to several emission features for all observer
orientations. For the hydrodynamical simulations considered we find both
ionization states and column densities for the outflowing gas that are too high
to be quantitatively consistent with well-observed X-ray absorption systems.
Nevertheless, our results are qualitatively encouraging and further exploration
of the model parameter space is warranted. (Abridged.)
Ultraviolet and X-ray observations show evidence of outflowing gas around
many active galactic nuclei. Some of these outflows may be driven off gas
infalling towards the central black hole. We perform radiative transfer
calculations to compute the gas ionization state and X-ray spectra for two- and
three-dimensional (3D) hydrodynamical simulations of this outflow-from-inflow
scenario. By comparison with observations, our results can be used to test the
theoretical models and guide future numerical simulations. We predict both
absorption and emission features, most of which are formed in a polar funnel of
outflowing gas. This outflow causes strong absorption for observer orientation
angles of < 35 degrees. Particularly in 3D, the strength of this absorption
varies significantly for different lines-of-sight owing to the fragmentary
structure of the gas flow. Although infalling material occupies a large
fraction of the simulation volume, we do not find that it imprints strong
absorption features since the ionization state is very high. Thus, an absence
of observed inflow absorption features does not exclude the models. The main
spectroscopic consequence of the infalling gas is a scattered continuum
component that partially re-fills the absorption features caused by the
outflowing polar funnel. Fluorescence and scattering in the outflow is
predicted to give rise to several emission features for all observer
orientations. For the hydrodynamical simulations considered we find both
ionization states and column densities for the outflowing gas that are too high
to be quantitatively consistent with well-observed X-ray absorption systems.
Nevertheless, our results are qualitatively encouraging and further exploration
of the model parameter space is warranted. (Abridged.)
Tuesday, July 31, 2012
Metal enrichment by radiation pressure in active galactic nucleus outflows -- theory and observations. (arXiv:1207.7038v1 [astro-ph.CO])
Metal enrichment by radiation pressure in active galactic nucleus outflows -- theory and observations. (arXiv:1207.7038v1 [astro-ph.CO]):
Outflows from active galactic nuclei may be produced by absorption of
continuum radiation by UV resonance lines of abundant metal ions, as observed
in broad absorption line quasars (BALQs). The radiation pressure exerted on the
metal ions is coupled to the rest of the gas through Coulomb collisions of the
metal ions. We calculate the photon density and gas density which allow
decoupling of the metal ions from the rest of the gas. These conditions may
lead to an outflow composed mostly of the metal ions. We derive a method to
constrain the metals/H ratio of observed UV outflows, based on the Ly {\alpha}
and Si iv {\lambda}{\lambda}1394, 1403 absorption profiles. We apply this
method to an SDSS sample of BALQs to derive a handful of candidate outflows
with a higher than solar metal/H ratio. This mechanism can produce ultra fast
UV outflows, if a shield of the continuum source with a strong absorption edge
is present.
Outflows from active galactic nuclei may be produced by absorption of
continuum radiation by UV resonance lines of abundant metal ions, as observed
in broad absorption line quasars (BALQs). The radiation pressure exerted on the
metal ions is coupled to the rest of the gas through Coulomb collisions of the
metal ions. We calculate the photon density and gas density which allow
decoupling of the metal ions from the rest of the gas. These conditions may
lead to an outflow composed mostly of the metal ions. We derive a method to
constrain the metals/H ratio of observed UV outflows, based on the Ly {\alpha}
and Si iv {\lambda}{\lambda}1394, 1403 absorption profiles. We apply this
method to an SDSS sample of BALQs to derive a handful of candidate outflows
with a higher than solar metal/H ratio. This mechanism can produce ultra fast
UV outflows, if a shield of the continuum source with a strong absorption edge
is present.
Thursday, July 26, 2012
Theoretical Uncertainties due to AGN Subgrid Models in Predictions of Galaxy Cluster Observable Properties. (arXiv:1207.6106v1 [astro-ph.CO])
Theoretical Uncertainties due to AGN Subgrid Models in Predictions of Galaxy Cluster Observable Properties. (arXiv:1207.6106v1 [astro-ph.CO]):
Cosmological constraints derived from galaxy clusters rely on accurate
predictions of cluster observable properties, in which feedback from active
galactic nuclei (AGN) is a critical component. In order to model the physical
effects due to supermassive black holes (SMBH) on cosmological scales, subgrid
modeling is required, and a variety of implementations have been developed in
the literature. However, theoretical uncertainties due to model and parameter
variations are not yet well understood, limiting the predictive power of
simulations including AGN feedback. By performing a detailed parameter
sensitivity study in a single cluster using several commonly-adopted AGN
accretion and feedback models with FLASH, we quantify the model uncertainties
in predictions of cluster integrated properties. We find that quantities that
are more sensitive to gas density have larger uncertainties (~20% for Mgas and
a factor of ~2 for Lx at R500), whereas Tx, Ysz, and Yx are more robust
(~10-20% at R500). To make predictions beyond this level of accuracy would
require more constraints on the most relevant parameters: the accretion model,
mechanical heating efficiency, and size of feedback region. By studying the
impact of AGN feedback on the scaling relations, we find that an
anti-correlation exists between Mgas and Tx, which is another reason why Ysz
and Yx are excellent mass proxies. This anti-correlation also implies that AGN
feedback is likely to be an important source of intrinsic scatter in the
Mgas-Tx and Lx-Tx relations.
Cosmological constraints derived from galaxy clusters rely on accurate
predictions of cluster observable properties, in which feedback from active
galactic nuclei (AGN) is a critical component. In order to model the physical
effects due to supermassive black holes (SMBH) on cosmological scales, subgrid
modeling is required, and a variety of implementations have been developed in
the literature. However, theoretical uncertainties due to model and parameter
variations are not yet well understood, limiting the predictive power of
simulations including AGN feedback. By performing a detailed parameter
sensitivity study in a single cluster using several commonly-adopted AGN
accretion and feedback models with FLASH, we quantify the model uncertainties
in predictions of cluster integrated properties. We find that quantities that
are more sensitive to gas density have larger uncertainties (~20% for Mgas and
a factor of ~2 for Lx at R500), whereas Tx, Ysz, and Yx are more robust
(~10-20% at R500). To make predictions beyond this level of accuracy would
require more constraints on the most relevant parameters: the accretion model,
mechanical heating efficiency, and size of feedback region. By studying the
impact of AGN feedback on the scaling relations, we find that an
anti-correlation exists between Mgas and Tx, which is another reason why Ysz
and Yx are excellent mass proxies. This anti-correlation also implies that AGN
feedback is likely to be an important source of intrinsic scatter in the
Mgas-Tx and Lx-Tx relations.
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