The Evolution of Normal Galaxy X-Ray Emission Through Cosmic History: Constraints from the 6 Ms Chandra Deep Field-South Page: 2
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THE ASTROPHYSICAL JOURNAL, 825:7 (24pp), 2016 July 1
cosmological simulation (Guo et al. 2011) and the Star-
track XRB population-synthesis code (e.g., Belczynski et al.
2002, 2008) to track the evolution of the stellar populations in
the universe and predict the X-ray emission associated with the
underlying XRB populations, respectively. The Fl3a models
follow the evolution of XRB populations and their parent
stellar populations throughout the history of the universe,
spanning z 20 to the present day, and provide predictions
for how the X-ray scaling relations (i.e., Lx(HMXB)/SFR and
Lx(LMXB)/M*) evolve with redshift. From this work, F13a
identified a "best fit" theoretical model that simultaneously fits
well both the observed Lx(HMXB)/SFR and Lx(LMXB)/M
scaling relations at z = 0. Subsequent observational tests have
shown that the F13a best-fit model provides reasonable
predictions for (1) the XRB luminosity functions of a sample
of nearby galaxies that had simple star-formation history
estimates from the literature (Tzanavaris et al. 2013); (2) the
metallicity dependence of the Lx(HMXB)/SFR relation for
powerful star-forming galaxies (Basu-Zych et al. 2013a;
Brorby et al. 2016); (3) the stellar-age dependence of the
Lx(LMXB)/M* relation for early-type galaxies (Lehmer et al.
2014; however, see Boroson et al. 2011 and Zhang et al. 2012);
(4) the redshift evolution out to z % 1.5 of the normal galaxy
X-ray luminosity functions in extragalactic Chandra surveys
(Tremmel et al. 2013); and (5) the redshift evolution of the total
Lx/SFR relation (i.e., using the summed HMXB plus LMXB
emission) for star-forming galaxies out to z N 4 (e.g., Basu-
Zych et al. 2013b).
The F13a theoretical modeling framework, as well as the
broad observational testing of its predictions, represent major
steps forward in our understanding of how XRBs form and
evolve along with their parent stellar populations. Within the
Fl3a framework, the most fundamental predictions include
prescriptions for how the Lx(HMXB)/SFR and Lx(LMXB)/
M, scaling relations evolve as a function of redshift (see Figure
5 of F13a). Due to sensitivity and angular-resolution limita-
tions, it is not possible to detect complete and representative
populations of cosmologically distant galaxies and separate
spatially their HMXB and LMXB contributions. However,
with deep (> 1 Ms) Chandra exposures and new multiwave-
length databases, several extragalactic surveys now have data
sufficient to isolate large populations of galaxies, measure their
global physical properties (e.g., SFR and M), and study their
population-averaged X-ray emission via stacking techniques
(see, e.g., Hornschemeier et al. 2002; Laird et al. 2006; Lehmer
et al. 2007, 2008; Cowie et al. 2012; Basu-Zych et al. 2013b).
With these advances, we can now conduct the most robust tests
to date of the F13a model predictions by examining the XRB
emission of galaxies dependence on SFR, M, and redshift.
The Chandra Deep Field-South (CDF-S) survey is the
deepest X-ray survey yet conducted. In this paper, we utilize
data products based on the first e6 Ms of data, which were
produced following the procedures outlined for the e4 Ms
exposure in Xue et al. (2011). An additional -1 Ms of data will
be added to the CDF-S, eventually bringing the total exposure
to -7 Ms; these results will be presented in Luo et al. (2016, in
preparation). In the e6 Ms exposure, 919 highly reliable X-ray
sources are detected to an ultimate 0.5-2 keV flux limit of
,7 x 10-18 erg cm-2 s-1, including 650 AGN candidates,
257 normal galaxy candidates, and 12 Galactic stars (see
Section 3 for classification details). For comparison, the 4 Ms
CDF-S catalog contained 740 sources down to an ultimate
LEHMER ET AL.
0.5-2 keV flux limit of _10--7 erg cm2 s-1, of which 568,
162, and 10 were classified as AGN, normal galaxies, and
Galactic stars. In the most sensitive regions of the survey field,
the 0.5-2 keV detected normal galaxies rival or exceed the
AGN in terms of source density (see, e.g., the Lehmer et al.
2012 analysis of the 4 Ms data).
Source catalogs based on optical/near-IR imaging contain
-25,000 galaxies within -7 arcmin of the CDF-S Chandra
aimpoint (e.g., Luo et al. 2011; Xue et al. 2012), indicating that
only a small fraction (<1%) of the known normal galaxy
population is currently detected in X-ray emission. In this
paper, we utilize X-ray stacking analyses of the galaxy
populations within the CDF-S, divided into redshift intervals
and subsamples of specific SFR, sSFR = SFR/M,, which is
an indicator of the ratio of HMXB-to-LMXB emission. These
measurements provide the first accounting of both HMXBs and
LMXBs at z > 0 to simultaneously constrain the evolution of
the Lx(HMXB)/SFR and Lx(LMXB)/M* scaling relations and
provide the most powerful and robust test of the F13a model
This paper is organized as follows: In Section 2, we define
our galaxy sample and describe our methods for measuring
SFR and M, values for the galaxies. In Section 3, we discuss
the X-ray properties of galaxies and scaling relations of our
sample that are individually detected in the -6 Ms images. In
Section 4, we detail our stacking procedure, and in Section 5
we define galaxy subsamples to be stacked. Results from our
stacking analyses, including characterizations of the evolution
of X-ray scaling relations, are presented in Section 6. Finally,
in Section 7, we interpret our results in the context of XRB
population-synthesis models, construct models of the evolution
of the X-ray emissivity of the universe, and estimate the
cosmic X-ray background contributions from normal galaxies.
Throughout this paper, we make use of the main point-
source catalog and data products for the -6 Ms CDF-S as will
be outlined in Luo et al. (2016, in preparation). The Luo et al.
(2016, in preparation) procedure is identical in nature to that
presented for the -4 Ms CDF-S in Xue et al. (2011). The
Galactic column density for the CDF-S is 8.8 x 1019 cm-2
(Stark et al. 1992). All of the X-ray fluxes and luminosities
quoted throughout this paper have been corrected for Galactic
absorption. Estimates of M, and SFR presented throughout this
paper have been derived assuming a Kroupa (2001) initial mass
function (IMF); when making comparisons with other studies,
we have adjusted all values to correspond to our adopted IMF.
Values of Ho 70 km s-- Mpc-1, QM = 0.3, and
QA = 0.7 are adopted throughout this paper.
2. GALAXY SAMPLE AND PHYSICAL PROPERTIES
We began with an initial sample of 32,508 galaxies in the
Great Observatories Origins Deep Survey South (GOODS-S)
footprint as presented in Section 2 of Xue et al. (2012;
hereafter X12; see also Luo et al. 2011). The X12 galaxy
sample was selected using the HST F850LP photometric data
from Dahlen et al. (2010), and contains objects down to a 5c-
limiting magnitude of Z850 N 28.1. We cut our initial sample to
the 24,941 objects that were within 7' of the mean -6 Ms
CDF-S aimpoint, a region where the Chandra point-spread
function (PSF) is sharpest and the corresponding X-ray
sensitivity is highest. Hereafter, we refer to the resulting
sample as our base sample. Of the 24,941 objects in our base
sample, 1,124 (4.5%) have secure spectroscopic redshifts. To
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Lehmer, Bret; Basu-Zych, A.; Mineo, S.; Brandt, William Nielsen; Eufrasio, R. T.; Fragos, T. et al. The Evolution of Normal Galaxy X-Ray Emission Through Cosmic History: Constraints from the 6 Ms Chandra Deep Field-South, article, June 24, 2016; Washington, D.C.. (https://digital.library.unt.edu/ark:/67531/metadc974443/m1/2/: accessed April 18, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT College of Arts and Sciences.