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