The X-ray star formation story as told by Lyman break galaxies in the 4 Ms CDF-S

We present results from deep X-ray stacking of >4000 high-redshift galaxies from z ≈ 1 to 8 using the 4 Ms Chandra Deep Field-South data, the deepest X-ray survey of the extragalactic sky to date. The galaxy samples were selected using the Lyman break technique based primarily on recent Hubble Space Telescope ACS and WFC3 observations. Based on such high specific star formation rates (sSFRs): log SFR/M_⊙* > -8.7, we expect that the observed properties of these Lyman break galaxies (LBGs) are dominated by young stellar populations. The X-ray emission in LBGs, eliminating individually detected X-ray sources (potential active galactic nucleus), is expected to be powered by X-ray binaries and hot gas. We find, for the first time, evidence of evolution in the X-ray/SFR relation. Based on X-ray stacking analyses for z < 4 LBGs (covering ∼90% of the universe's history), we find that the 2-10 keV X-ray luminosity evolves weakly with redshift (z) and SFR as log L _X = 0.93log (1 + z) + 0.65log SFR + 39.80. By comparing our observations with sophisticated X-ray binary population synthesis models, we interpret that the redshift evolution of L _X/SFR is driven by metallicity evolution in high mass X-ray binaries, likely the dominant population in these high sSFR galaxies. We also compare these models with our observations of X-ray luminosity density (total 2-10 keV luminosity per Mpc³) and find excellent agreement. While there are no significant stacked detections at z ≳ 5, we use our upper limits from 5 ≲ z ≲ 8 LBGs to constrain the supermassive black hole accretion history of the universe around the epoch of reionization.