Heavily Obscured AGN in High Redshift Luminous Infrared Galaxies

TL;DR

This study uses deep multi-wavelength data from the Chandra Deep Field South to identify heavily-obscured low-luminosity AGN in infrared-luminous galaxies, revealing their significant contribution to black hole growth and their prevalence in high-mass galaxies.

Contribution

It provides the first stacked X-ray spectral analysis of IR-luminous galaxies, demonstrating the presence and significance of obscured AGN in these systems, especially in high-mass galaxies.

Findings

Heavily-obscured AGN are more common in luminous IR and high-mass galaxies.

Approximately 22% of black hole growth occurs in obscured, undetected systems.

Low-luminosity, obscured AGN show little evolution with redshift.

Abstract

We take advantage of the rich multi-wavelength data available in the Chandra Deep Field South (CDF-S), including the 4 Msec Chandra observations (the deepest X-ray data to date), in order to search for heavily-obscured low-luminosity AGN among infrared-luminous galaxies. In particular, we obtained a stacked rest-frame X-ray spectrum for samples of galaxies binned in terms of their IR luminosity or stellar mass. We detect a significant signal at E~1 to 8 keV, which we interpret as originating from a combination of emission associated with star-formation processes at low energies combined with a heavily-obscured AGN at E>5 keV. We further find that the relative strength of this AGN signal decays with decreasing IR luminosity, indicating a higher AGN fraction for more luminous IR sources. Together, these results strongly suggest the presence of a large number of obscured AGN in IR-luminous galaxies. Using samples binned in terms of stellar mass in the host galaxy, we find a significant excess at E=6-7 keV for sources with M>10^{11} Msun, consistent with a large obscured AGN population in high mass galaxies. In contrast, no strong evidence of AGN activity was found for less-massive galaxies. The integrated intensity at high energies indicates that a significant fraction of the total black hole growth, ~22%, occurs in heavily-obscured systems that are not individually detected in even the deepest X-ray observations. There are also indications that the number of low-luminosity, heavily-obscured AGN does not evolve significantly with redshift, in contrast to the strong evolution seen in higher luminosity sources.