  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.
