The X-ray Star Formation Story as Told by Lyman Break Galaxies in the 4 Ms CDF-S. (arXiv:1210.3357v1 [astro-ph.CO])

The X-ray Star Formation Story as Told by Lyman Break Galaxies in the 4 Ms CDF-S. (arXiv:1210.3357v1 [astro-ph.CO]):
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 (CDF-S) 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 HST 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 LBGs are
dominated by young stellar populations. The X-ray emission in LBGs, eliminating
individually detected X-ray sources (potential AGN), 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 LX = 0.93 log (1+z)
+ 0.65 log SFR + 39.80. By comparing our observations with sophisticated X-ray
binary population synthesis models, we interpret that the redshift evolution of
LX/SFR is driven by metallicity evolution in HMXBs, 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 Mpc^3)
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 SMBH accretion
history of the Universe around the epoch of reionization.