536 research outputs found
Can the unresolved X-ray background be explained by emission from the optically-detected faint galaxies of the GOODS project?
The emission from individual X-ray sources in the Chandra Deep Fields and
XMM-Newton Lockman Hole shows that almost half of the hard X-ray background
above 6 keV is unresolved and implies the existence of a missing population of
heavily obscured active galactic nuclei (AGN). We have stacked the 0.5-8 keV
X-ray emission from optical sources in the Great Observatories Origins Deep
Survey (GOODS; which covers the Chandra Deep Fields) to determine whether these
galaxies, which are individually undetected in X-rays, are hosting the
hypothesised missing AGN. In the 0.5-6 keV energy range the stacked-source
emission corresponds to the remaining 10-20 per cent of the total background --
the fraction that has not been resolved by Chandra. The spectrum of the stacked
emission is consistent with starburst activity or weak AGN emission. In the 6-8
keV band, we find that upper limits to the stacked X-ray intensity from the
GOODS galaxies are consistent with the ~40 per cent of the total background
that remains unresolved, but further selection refinement is required to
identify the X-ray sources and confirm their contribution.Comment: 7 pages, 1 figure, accepted for publication in MNRA
Evidence for a constant IMF in early-type galaxies based on their X-ray binary populations
A number of recent studies have proposed that the stellar initial mass
function (IMF) of early type galaxies varies systematically as a function of
galaxy mass, with higher mass galaxies having bottom heavy IMFs. These bottom
heavy IMFs have more low-mass stars relative to the number of high mass stars,
and therefore naturally result in proportionally fewer neutron stars and black
holes. In this paper, we specifically predict the variation in the number of
black holes and neutron stars based on the power-law IMF variation required to
reproduce the observed mass-to-light ratio trends with galaxy mass. We then
test whether such variations are observed by studying the field low-mass X-ray
binary populations (LMXBs) of nearby early-type galaxies. In these binaries, a
neutron star or black hole accretes matter from a low-mass donor star. Their
number is therefore expected to scale with the number of black holes and
neutron stars present in a galaxy. We find that the number of LMXBs per K-band
light is similar among the galaxies in our sample. These data therefore
demonstrate the uniformity of the slope of the IMF from massive stars down to
those now dominating the K-band light, and are consistent with an invariant
IMF. Our results are inconsistent with an IMF which varies from a
Kroupa/Chabrier like IMF for low mass galaxies to a steep power-law IMF (with
slope =2.8) for high mass galaxies. We discuss how these observations
constrain the possible forms of the IMF variations and how future Chandra
observations can enable sharper tests of the IMF.Comment: 12 pages, 5 figures, 2 tables, submitted to Ap
X-ray emission from star-forming galaxies - III. Calibration of the Lx-SFR relation up to redshift z1.3
We investigate the relation between total X-ray emission from star-forming
galaxies and their star formation activity. Using nearby late-type galaxies and
ULIRGs from Paper I and star-forming galaxies from Chandra Deep Fields, we
construct a sample of 66 galaxies spanning the redshift range z~0-1.3 and the
star-formation rate (SFR) range ~0.1-10^3 M_sun/yr. In agreement with previous
results, we find that the Lx-SFR relation is consistent with a linear law both
at z=0 and for the z=0.1-1.3 CDF galaxies, within the statistical accuracy of
~0.1 in the slope of the Lx-SFR relation. For the total sample, we find a
linear scaling relation Lx/SFR~(4.0\pm 0.4)x10^{39}(erg/s)/(Msun/yr), with a
scatter of ~0.4 dex. About ~2/3 of the 0.5-8 keV luminosity generated per unit
SFR is expected to be due to HMXBs. We find no statistically significant trends
in the mean Lx/SFR ratio with the redshift or star formation rate and constrain
the amplitude of its variations by <0.1-0.2 dex. These properties make X-ray
observations a powerful tool to measure the star formation rate in normal
star-forming galaxies that dominate the source counts at faint fluxes.Comment: 11 pages, 3 tables, 4 figures, accepted for publication by MNRAS.
Substantial changes since the last version, including the authors lis
Pseudorandom Number Generators and the Square Site Percolation Threshold
A select collection of pseudorandom number generators is applied to a Monte
Carlo study of the two dimensional square site percolation model. A generator
suitable for high precision calculations is identified from an application
specific test of randomness. After extended computation and analysis, an
ostensibly reliable value of pc = 0.59274598(4) is obtained for the percolation
threshold.Comment: 11 pages, 6 figure
Testing the Universality of the Stellar IMF with Chandra and HST
The stellar initial mass function (IMF), which is often assumed to be
universal across unresolved stellar populations, has recently been suggested to
be "bottom-heavy" for massive ellipticals. In these galaxies, the prevalence of
gravity-sensitive absorption lines (e.g. Na I and Ca II) in their near-IR
spectra implies an excess of low-mass ( ) stars over that
expected from a canonical IMF observed in low-mass ellipticals. A direct
extrapolation of such a bottom-heavy IMF to high stellar masses (
) would lead to a corresponding deficit of neutron stars and black
holes, and therefore of low-mass X-ray binaries (LMXBs), per unit near-IR
luminosity in these galaxies. Peacock et al. (2014) searched for evidence of
this trend and found that the observed number of LMXBs per unit -band
luminosity () was nearly constant. We extend this work using new and
archival Chandra X-ray Observatory (Chandra) and Hubble Space Telescope (HST)
observations of seven low-mass ellipticals where is expected to be the
largest and compare these data with a variety of IMF models to test which are
consistent with the observed . We reproduce the result of Peacock et al.
(2014), strengthening the constraint that the slope of the IMF at
must be consistent with a Kroupa-like IMF. We construct an IMF model
that is a linear combination of a Milky Way-like IMF and a broken power-law
IMF, with a steep slope ( ) for stars < 0.5 (as
suggested by near-IR indices), and that flattens out ( ) for
stars > 0.5 , and discuss its wider ramifications and limitations.Comment: Accepted for publication in ApJ; 7 pages, 2 figures, 1 tabl
Evidence for Elevated X-ray Emission in Local Lyman Break Galaxy Analogs
In this paper, we study the relationship between the 2-10 keV X-ray
luminosity (L_X), assumed to originate from X-ray binaries (XRBs), and star
formation rate (SFR) in UV-selected z<0.1 Lyman break analogs (LBAs). We
present Chandra observations for four new GALEX-selected LBAs. Including
previously studied LBAs, Haro 11 and VV 114, we find that LBAs demonstrate
L_X/SFR ratios that are elevated by ~1.5sigma compared to local galaxies,
similar to the ratios found for stacked LBGs in the early Universe (z>2). We
show that these LBAs are unlikely to harbor AGN, based on their optical and
X-ray spectra and the spatial distribution of the X-rays in three spatially
extended cases. We expect that high-mass X-ray binaries (HMXBs) dominate the
X-ray emission in these galaxies, based on their high specific SFRs
(sSFRs=SFR/M* > 10^{-9}/yr), which suggest the prevalence of young stellar
populations. Since both LBAs and LBGs have lower dust attenuations and
metallicities compared to similar samples of more typical local galaxies, we
investigate the effects of dust extinction and metallicity on the L_X/SFR for
the broader population of galaxies with high sSFRs (>10^{-10}/yr). The
estimated dust extinctions (corresponding to column densities of
N_H<10^{22}/cm^2) are expected to have insignificant effects on observed
L_X/SFR ratio for the majority of galaxy samples. We find that the observed
relationship between L_X/SFR and metallicity appears consistent with
theoretical expectations from X-ray binary population synthesis models.
Therefore, we conclude that lower metallicities, related to more luminous HMXBs
such as ultraluminous X-ray sources (ULXs), drive the elevated L_X/SFR observed
in our sample of z<0.1 LBAs. The relatively metal-poor, active mode of star
formation in LBAs and distant z>2 LBGs may yield higher total HMXB luminosity
than found in typical galaxies in the local Universe.Comment: 11 pages, 7 figures, Submitted to ApJ (references updated in v2
The X-ray Properties of the Nearby Star-Forming Galaxy IC 342: The XMM-Newton View
We present the X-ray properties of IC342 using XMM-Newton. Thirty-five
sources are detected coincident with the disk of IC342 (more than tripling the
number known), of which ~31 are likely to be intrinsic to IC342. This
population shows a range of spectral properties and has an X-ray luminosity
function slope and infrared luminosity comparable to that of starburst galaxies
such as M82 and the Antennae, while its relative lack of extended X-ray
emission is similar to the properties of quiescent spirals. We do detect
long-term variability between this observation and the 1991 ROSAT and 1993/2000
ASCA observations for five sources. Notably, the second most luminous source
IC342 X-2 is is found to be in its the lowest luminosity state observed for X-2
to date, although the slope of the spectrum is intermediate between the
previously observed low/hard and high/soft states. IC342 X-1, on the other
hand, is found to be in an identical state to that observed in 2000 with ASCA.
Assuming X-1 is in an anomalous very high (VH) state, then either (1) X-1 has
remained in this state between 2000 and 2002, and is therefore the longest
duration VH-state binary ever observed, or (2) it was simply caught in a VH
state by chance in both the 2000 ASCA and 2002 XMM-Newton observations. We have
also confirmed the ROSAT HRI result that the nucleus of IC342 is made up of
both point-like and extended emission. The relative fluxes of the two spectral
components suggest that the nucleus is complex, with a soft extended component
contributing approximately half of the total luminosity. (Abridged)Comment: AJ in press (December 2003), 9 pages, 7 figures, 2 tables,
emulateapj.cls use
- …