The X-ray Star Formation Story as Told by Lyman Break Galaxies in the 4 Ms CDF-S

TL;DR

This study uses deep X-ray stacking of over 4000 high-redshift Lyman break galaxies from z~1 to 8 to investigate their X-ray emission, revealing evolution in the X-ray/SFR relation driven by metallicity changes in high-mass X-ray binaries.

Contribution

First evidence of evolution in the X-ray/SFR relation for high-redshift galaxies, linking it to metallicity evolution in HMXBs, based on deep stacking analysis of LBGs from z~1 to 8.

Findings

X-ray luminosity evolves weakly with redshift and SFR.

X-ray emission is dominated by X-ray binaries and hot gas, not AGN.

Constraints on SMBH accretion history at z>5.

Abstract

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.