Tracking Down the Source Population Responsible for the Unresolved Cosmic 6-8 keV Background

TL;DR

This study identifies a population of low-mass, highly obscured AGN-hosting galaxies at redshifts 1-3 that significantly contribute to the unresolved 6-8 keV cosmic X-ray background, revealing a hidden source population.

Contribution

It uncovers a previously unrecognized population of low-mass, obscured AGNs responsible for the unresolved X-ray background, expanding understanding of cosmic X-ray sources.

Findings

6845 galaxies contribute to unresolved XRB

Stacked detection significant at 3.9 sigma in 6-8 keV band

Major contributors are low-mass, blue cloud galaxies at z=1-3

Abstract

Using the 4 Ms Chandra Deep Field-South (CDF-S) survey, we have identified a sample of 6845 X-ray undetected galaxies that dominates the unresolved ~ 20-25% of the 6-8 keV cosmic X-ray background (XRB). This sample was constructed by applying mass and color cuts to sources from a parent catalog based on GOODS-South HST z-band imaging of the central 6'-radius area of the 4 Ms CDF-S. The stacked 6-8 keV detection is significant at the 3.9 sigma level, but the stacked emission was not detected in the 4-6 keV band which indicates the existence of an underlying population of highly obscured active galactic nuclei (AGNs). Further examinations of these 6845 galaxies indicate that the galaxies on the top of the blue cloud and with redshifts of 1 < z < 3, magnitudes of 25 < z_850 < 28, and stellar masses of 2E8 < M_star/M_sun < 2E9 make the majority contributions to the unresolved 6-8 keV XRB. Such a population is seemingly surprising given that the majority of the X-ray detected AGNs reside in massive (> ~1E10 M_sun) galaxies. We discuss constraints upon this underlying AGN population, supporting evidence for relatively low-mass galaxies hosting highly obscured AGNs, and prospects for further boosting the stacked signal.