The Stellar Age Dependence of X-ray Emission from Normal Star-Forming
  Galaxies in the GOODS Fields
  The Chandra Deep Field-South and North surveys (CDFs) provide unique windows
into the cosmic history of X-ray emission from normal (non-active) galaxies.
Scaling relations of normal galaxy X-ray luminosity (L_X) with star formation
rate (SFR) and stellar mass (M_star) have been used to show that the formation
rates of low-mass and high-mass X-ray binaries (LMXBs and HMXBs, respectively)
evolve with redshift across z = 0-2 following L_HMXB/SFR ~ 1 + z and
L_LMXB/M_star ~ (1 + z)^{2-3}. However, these measurements alone do not
directly reveal the physical mechanisms behind the redshift evolution of X-ray
binaries (XRBs). We derive star-formation histories for a sample of 344 normal
galaxies in the CDFs, using spectral energy distribution (SED) fitting of
FUV-to-FIR photometric data, and construct a self-consistent, age-dependent
model of the X-ray emission from the galaxies. Our model quantifies how X-ray
emission from hot gas and XRB populations vary as functions of host
stellar-population age. We find that (1) the ratio L_X/M_star declines by a
factor of ~1000 from 0-10 Gyr and (2) the X-ray SED becomes harder with
increasing age, consistent with a scenario in which the hot gas contribution to
the X-ray SED declines quickly for ages above 10 Myr. When dividing our sample
into subsets based on metallicity, we find some indication that L_X/M_star is
elevated for low-metallicity galaxies, consistent with recent studies of X-ray
scaling relations. However, additional statistical constraints are required to
quantify both the age and metallicity dependence of X-ray emission from
star-forming galaxies.
