2016MNRAS.457.4081B

The source of energetic photons that heated and reionized the early Universe remains uncertain. Early galaxies had low metallicity and recent population synthesis calculations suggest that the number and luminosity of high-mass X-ray binaries are enhanced in star-forming galaxies with low metallicity, offering a potentially important and previously overlooked source of heating and reionization. Lyman break analogue (LBA) galaxies are local galaxies that strongly resemble the high-redshift, star-forming Lyman break galaxies and have been suggested as local analogues to these metal-deficient galaxies found in the early Universe. We studied a sample of 10 LBAs in order to measure the relation between star formation rate and X-ray luminosity. We found that for LBAs with metallicities in the range 12 + log₁₀(O/H)=8.15-8.80, the L_X -SFR relation was in the 0.5-8 keV band with a dispersion of σ=0.25 dex. This is an enhancement of nearly a factor of 2 in the L_0.5-8 keV_-SFR relation relative to results for nearby, near-solar metallicity galaxies. The enhancement is significant at the 98.2 per cent level (2.4σ). Our enhanced L_X/SFR relation is consistent with the metallicity-dependent predicted value from population synthesis models. We discuss the possibility of an L_X-SFR-metallicity plane for star-forming galaxies. These results are important to our understanding of reionization and the formation of early galaxies.