The Metallicity Dependence of the High-Mass X-ray Binary Luminosity
  Function
  We present detailed constraints on the metallicity dependence of the high
mass X-ray binary (HMXB) X-ray luminosity function (XLF). We analyze ~5 Ms of
Chandra data for 55 actively star-forming galaxies at D < 30 Mpc with gas-phase
metallicities spanning 12 + log(O/H) = 7-9.2. Within the galactic footprints,
our sample contains a total of 1311 X-ray point sources, of which ~49% are
expected to be HMXBs, with the remaining sources likely to be low-mass X-ray
binaries (LMXBs; ~22%) and unrelated background sources (~29%). We construct a
model that successfully characterizes the average HMXB XLF over the full
metallicity range. We demonstrate that the SFR-normalized HMXB XLF shows clear
trends with metallicity, with steadily increasing numbers of luminous and
ultraluminous X-ray sources (logL(erg/s) = 38-40.5) with declining metallicity.
However, we find that the low-luminosity (logL(erg/s) = 36-38) HMXB XLF appears
to show a nearly constant SFR scaling and slope with metallicity. Our model
provides a revised scaling relation of integrated LX/SFR versus 12 + log(O/H)
and a new characterization of its the SFR-dependent stochastic scatter. The
general trend of this relation is broadly consistent with past studies based on
integrated galaxy emission; however, our model suggests that this relation is
driven primarily by the high-luminosity end of the HMXB XLF. Our results have
implications for binary population synthesis models, the nature of
super-Eddington accreting objects (e.g., ultraluminous X-ray sources), recent
efforts to identify active galactic nucleus candidates in dwarf galaxies, and
the X-ray radiation fields in the early Universe during the epoch of cosmic
heating at z > 10.
