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.

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.

Population III Stars and Remnants in High Redshift Galaxies. (arXiv:1305.1325v1 [astro-ph.CO])

Population III Stars and Remnants in High Redshift Galaxies. (arXiv:1305.1325v1 [astro-ph.CO]):
Recent simulations of Population III star formation have suggested that some
fraction form in binary systems, in addition to having a characteristic mass of
tens of solar masses. The deaths of metal-free stars result in the initial
chemical enrichment of the universe and the production of the first
stellar-mass black holes. Here we present a cosmological adaptive mesh
refinement simulation of an overdense region that forms a few 10^9 Msun dark
matter halos and over 13,000 Population III stars by redshift 15. We find that
most halos do not form Population III stars until they reach Mvir ~ 10^7 Msun
because this biased region is quickly enriched from both Population III and
galaxies, which also produce high levels of ultraviolet radiation that suppress
H2 formation. Nevertheless, Population III stars continue to form, albeit in
more massive halos, at a rate of ~ 10^{-4} Msun yr^{-1} Mpc^{-3} at redshift
15. The most massive starless halo has a mass of 7 x 10^7 Msun, which could
host massive black hole formation through the direct gaseous collapse scenario.
We show that the multiplicity of the Population III remnants grows with halo
mass above 10^8 Msun, culminating in 50 remnants located in 10^9 Msun halos on
average. This has implications that high mass X-ray binaries and intermediate
mass black holes that originate from metal-free stars may be abundant in
high-redshift galaxies.

Cosmological growth and feedback from supermassive black holes. (arXiv:1305.0286v1 [astro-ph.CO])

Cosmological growth and feedback from supermassive black holes. (arXiv:1305.0286v1 [astro-ph.CO]):
We develop a simple evolutionary scenario for the growth of supermassive
black holes (BHs), assuming growth due to accretion only, to learn about the
evolution of the BH mass function from $z=3$ to 0 and from it calculate the
energy budgets of different modes of feedback. We tune the parameters of the
model by matching the derived X-ray luminosity function (XLF) with the observed
XLF of active galactic nuclei. We then calculate the amount of comoving kinetic
and bolometric feedback as a function of redshift, derive a kinetic luminosity
function and estimate the amount of kinetic feedback and $PdV$ work done by
classical double Fanaroff-Riley II (FR II) radio sources. We also derive the
radio luminosity function for FR IIs from our synthesized population and set
constraints on jet duty cycles. Around 1/6 of the jet power from FR II sources
goes into $PdV$ work done in the expanding lobes during the time the jet is on.
Anti hierarchical growth of BHs is seen in our model due to addition of an
amount of mass being accreted on to all BHs independent of the BH mass. The
contribution to the total kinetic feedback by active galaxies in a low
accretion, kinetically efficient mode is found to be the most significant at
$z<1.5$. FR II feedback is found to be a significant mode of feedback above
redshifts $z\sim 1.5$, which has not been highlighted by previous studies.

The Structure of the Milky Way's Hot Gas Halo. (arXiv:1305.2430v1 [astro-ph.GA])

The Structure of the Milky Way's Hot Gas Halo. (arXiv:1305.2430v1 [astro-ph.GA]):
The Milky Way's million degree gaseous halo contains a considerable amount of
mass that, depending on its structural properties, can be a significant mass
component. In order to analyze the structure of the Galactic halo, we use
XMM-Newton RGS archival data and measure OVII K alpha absorption-line strengths
towards 26 active galactic nuclei (AGN), LMC X-3, and two Galactic sources (4U
1820-30 and X1735-444). We assume a beta-model as the underlying gas density
profile and find best-fit parameters of n_o = 0.46^{+0.74}_{-0.35} cm^-3, r_c =
0.35^{+0.29}_{-0.27} kpc, and beta = 0.71^{+0.13}_{-0.14}. These parameters
result in halo masses ranging between M(18 kpc) = 7.5^{+22.0}_{-4.6} x 10^8
M_sun and M(200 kpc) = 3.8^{+6.0}_{-0.5} x 10^{10} M_sun assuming a gas
metallicity of Z = 0.3 Z_sun, which are consistent with current theoretical and
observational work. The maximum baryon fraction from our halo model of f_b =
0.07^{+0.03}_{-0.01} is significantly smaller than the universal value of f_b =
0.171, implying the mass contained in the Galactic halo accounts for 10 - 50%
of the missing baryons in the Milky Way. We also discuss our model in the
context of several Milky Way observables, including ram pressure stripping in
dwarf spheroidal galaxies, the observed X-ray emission measure in the 0.5 - 2
keV band, the Milky Way's star formation rate, spatial and thermal properties
of cooler gas (~10^5 K) and the observed Fermi bubbles towards the Galactic
center. Although the metallicity of the halo gas is a large uncertainty in our
analysis, we place a lower limit on the halo gas between the Sun and the LMC.
We find that Z >~ 0.2 Z_sun based on the pulsar dispersion measure towards the
LMC.

Neutron Star Masses and Radii from Quiescent Low-Mass X-ray Binaries. (arXiv:1305.3242v1 [astro-ph.HE])

Neutron Star Masses and Radii from Quiescent Low-Mass X-ray Binaries. (arXiv:1305.3242v1 [astro-ph.HE]):
A recent analysis (Guillot et al. 2013) of the thermal spectra of 5 quiescent
low-mass X-ray binaries in globular clusters, in which it was assumed that all
neutron stars have the same radius, determined the radius to be
R=9.1^{+1.3}_{-1.5} km to 90% confidence. However, the masses of the sources
were found to range from 0.86 solar masses to 2.4 solar masses and a
significant amount of the predicted M-R region violates causality and the
existence of a 2 solar mass neutron star. The study determined the amount of
Galactic absorption along the lines-of-sight from fitting the X-ray spectra and
assumed all sources possessed hydrogen atmospheres. We argue, from a Bayesian
analysis, that different interpretations of the data are strongly favored. Our
most-favored model assumes i) the equation of state of neutron star crusts is
well-understood, ii) the high-density equation of state is consistent with
causality and the existence of neutron stars at least as massive as 2 solar
masses, iii) that the Galactic absorption is determined either from the fits in
Guillot et al. (2013) or from independent HI surveys, and iv) that these
objects are well-described by either hydrogen or helium atmospheres. With these
assumptions, the 90% confidence radius range for 1.4 solar mass stars is 11.4
to 12.8 km, and the allowed range for radii of all neutron stars between 1.2
solar masses and 2.0 solar masses is 10.9 to 12.7 km. This result is in much
greater agreement with predictions of the equation of state from both nuclear
experiments and theoretical neutron matter studies than the smaller radii
deduced by Guillot et al. (2013).

The Growth of Cool Cores and Evolution of Cooling Properties in a Sample of 83 Galaxy Clusters at 0.3 < z < 1.2 Selected from the SPT-SZ Survey. (arXiv:1305.2915v1 [astro-ph.CO])

The Growth of Cool Cores and Evolution of Cooling Properties in a Sample of 83 Galaxy Clusters at 0.3 < z < 1.2 Selected from the SPT-SZ Survey. (arXiv:1305.2915v1 [astro-ph.CO]):
We present first results on the cooling properties derived from Chandra X-ray
observations of 83 high-redshift (0.3 < z < 1.2) massive galaxy clusters
selected by their Sunyaev-Zel'dovich signature in the South Pole Telescope
data. We measure each cluster's central cooling time, central entropy, and mass
deposition rate, and compare to local cluster samples. We find no significant
evolution from z~0 to z~1 in the distribution of these properties, suggesting
that cooling in cluster cores is stable over long periods of time. We also find
that the average cool core entropy profile in the inner ~100 kpc has not
changed dramatically since z ~ 1, implying that feedback must be providing
nearly constant energy injection to maintain the observed "entropy floor" at
~10 keV cm^2. While the cooling properties appear roughly constant over long
periods of time, we observe strong evolution in the gas density profile, with
the normalized central density (rho_0/rho_crit) increasing by an order of
magnitude from z ~ 1 to z ~ 0. When using metrics defined by the inner surface
brightness profile of clusters, we find an apparent lack of classical, cuspy,
cool-core clusters at z > 0.75, consistent with earlier reports for clusters at
z > 0.5 using similar definitions. Our measurements indicate that cool cores
have been steadily growing over the 8 Gyr spanned by our sample, consistent
with a constant, ~150 Msun/yr cooling flow that is unable to cool below
entropies of 10 keV cm^2 and, instead, accumulates in the cluster center. We
estimate that cool cores began to assemble in these massive systems at z ~ 1,
which represents the first constraints on the onset of cooling in galaxy
cluster cores. We investigate several potential biases which could conspire to
mimic this cool core evolution and are unable to find a bias that has a similar
redshift dependence and a substantial amplitude.