Metallicity dependence of HMXB populations
  High-mass X-ray binaries (HMXBs) might have contributed a non-negligible
fraction of the energy feedback to the interstellar and intergalactic media at
high redshift, becoming important sources for the heating and ionization
history of the Universe. However, the importance of this contribution depends
on the hypothesized increase in the number of HMXBs formed in low-metallicity
galaxies and in their luminosities. In this work we test the aforementioned
hypothesis, and quantify the metallicity dependence of HMXB population
properties. We compile from the literature a large set of data on the sizes and
X-ray luminosities of HMXB populations in nearby galaxies with known
metallicities and star formation rates. We use Bayesian inference to fit simple
Monte Carlo models that describe the metallicity dependence of the size and
luminosity of the HMXB populations. We find that HMXBs are typically ten times
more numerous per unit star formation rate in low-metallicity galaxies (12 +
log(O/H) < 8, namely < 20% solar) than in solar-metallicity galaxies. The
metallicity dependence of the luminosity of HMXBs is small compared to that of
the population size. Our results support the hypothesis that HMXBs are more
numerous in low-metallicity galaxies, implying the need to investigate the
feedback in the form of X-rays and energetic mass outflows of these high-energy
sources during cosmic dawn.
