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Heavily Obscured AGN in High Redshift Luminous Infrared Galaxies

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.