Enhanced X-ray emission from Lyman Break Analogues and a Possible $L_{\rm X}$--SFR--Metallicity Plane

TL;DR

This study investigates the X-ray emission in local Lyman Break Analogue galaxies, revealing an enhanced relation between X-ray luminosity and star formation rate that depends on metallicity, informing models of early galaxy evolution and reionization.

Contribution

The paper provides the first measurement of the $L_X$-SFR relation in LBAs, showing a metallicity-dependent enhancement consistent with population synthesis predictions.

Findings

LBA galaxies show nearly double the $L_X$/SFR compared to higher metallicity galaxies.

The $L_X$-SFR relation has a dispersion of 0.25 dex.

Results support a possible $L_X$-SFR-metallicity plane for star-forming galaxies.

Abstract

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 ten 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 $12+\log_{10}({\rm O/H}) = 8.15-8.80$, the $L_X-$SFR relation was $\log_{10} (L_X/{\rm SFR}\, {[\rm erg\ s^{-1}\ M_{\odot}^{-1}\ yr]}) = 39.85(\pm 0.10)$ in the $0.5-8$~keV band with a dispersion of $\sigma = 0.25$~dex. This is an enhancement of nearly a factor of $2$ in the $L_{0.5-8\text{keV}}$--SFR relation relative to results for nearby, near-solar metallicity galaxies. The enhancement is significant at the 98.2\% level ($2.4\sigma$). Our enhanced $L_X/{\rm SFR}$ relation is consistent with the metallicity-dependent predicted value from population synthesis models. We discuss the possibility of a $L_X$--SFR--Metallicity plane for star-forming galaxies. These results are important to our understanding of reionization and the formation of early galaxies.