Effects of Metallicity on High Mass X-ray Binary Formation
  The heating of the intergalactic medium in the early, metal-poor Universe may
have been partly due to radiation from high mass X-ray binaries (HMXBs).
Previous investigations on the effect of metallicity have used galaxies of
different types. To isolate the effects of metallicity on the production of
HMXBs, we study a sample consisting only of 46 blue compact dwarf galaxies
(BCDs) covering metallicity in the range 12+log(O/H) of 7.15 to 8.66. To test
the hypothesis of metallicity dependence in the X-ray luminosity function
(XLF), we fix the XLF form to that found for near-solar metallicity galaxies
and use a Bayesian method to constrain the XLF normalization as a function of
star formation rate (SFR) for three different metallicity ranges in our sample.
We find an increase by a factor of 4.45 $\pm$ 2.04 in the XLF normalization
between the metallicity ranges 7.1-7.7 and 8.2-8.66 at a statistical
significance of 99.79 per cent. Our results suggest that HMXB production is
enhanced at low metallicity, and consequently that HMXBs may have contributed
significantly to the reheating of the early Universe.
