X-ray binary formation in low-metallicity blue compact dwarf galaxies
  X-rays from binaries in small, metal-deficient galaxies may have contributed
significantly to the heating and reionization of the early universe. We
investigate this claim by studying blue compact dwarfs (BCDs) as local
analogues to these early galaxies. We constrain the relation of the X-ray
luminosity function (XLF) to the star-formation rate (SFR) using a Bayesian
approach applied to a sample of 25 BCDs. The functional form of the XLF is
fixed to that found for near-solar metallicity galaxies and is used to find the
probability distribution of the normalisation that relates X-ray luminosity to
SFR. Our results suggest that the XLF normalisation for low metallicity BCDs
(12+log(O/H) < 7.7) is not consistent with the XLF normalisation for galaxies
with near solar metallicities, at a confidence level of 1-5E-6. The XLF
normalisation for the BCDs is found to be 14.5 +/- 4.8 (M_solar^-1 yr), a
factor of 9.7 +/- 3.2 higher than for near solar metallicity galaxies.
Simultaneous determination of the XLF normalisation and power law index result
in estimates of q = 21.2^+12.2_-8.8) (M_solar^-1 yr) and alpha =
1.89^{+0.41}_{-0.30}, respectively. Our results suggest a significant
enhancement in the population of high-mass X-ray binaries in BCDs compared to
the near-solar metallicity galaxies. This suggests that X-ray binaries could
have been a significant source of heating in the early universe.
