Enhanced X-ray emission from Lyman break analogues and a possible LX-SFR-metallicity plane

Abstract

The source of energetic photons that heated and reionized the early Universe emains 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 Xray luminosity. We found that for LBAs with metallicities in the range 12 + log10(O/H) =8.15–8.80, the LX −SFR relation was log10(LX/SFR [erg s−1 M−1 yr]) = 39.85(±0.10) 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 L0.5−8 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 LX/SFR relation is consistent with the metallicity-dependent predicted value from population synthesis models. We discuss the possibility of an LX–SFR–metallicity plane for star-forming galaxies. These results are important to our understanding of reionization and the formation of early galaxies.