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 .