Luminous buried AGNs as a function of galaxy infrared luminosity
  revealed through Spitzer low-resolution infrared spectroscopy
  We present the results of Spitzer IRS infrared 5-35 micron low-resolution
spectroscopic energy diagnostics of ultraluminous infrared galaxies (ULIRGs) at
z > 0.15, classified optically as non-Seyferts. Based on the equivalent widths
of polycyclic aromatic hydrocarbon emission and the optical depths of silicate
dust absorption features, we searched for signatures of intrinsically luminous,
but optically elusive, buried AGNs in these optically non-Seyfert ULIRGs. We
then combined the results with those of non-Seyfert ULIRGs at z < 0.15 and
non-Seyfert galaxies with infrared luminosities L(IR) < 10^12Lsun. We found
that the energetic importance of buried AGNs clearly increases with galaxy
infrared luminosity, becoming suddenly discernible in ULIRGs with L(IR) >
10{12}Lsun. For ULIRGs with buried AGN signatures, a significant fraction of
infrared luminosities can be accounted for by detected buried AGN and
modestly-obscured (Av < 20 mag) starburst activity. The implied masses of
spheroidal stellar components in galaxies for which buried AGNs become
important roughly correspond to the value separating red massive and blue,
less-massive galaxies in the local universe. Our results may support the
widely-proposed AGN-feedback scenario as the origin of galaxy downsizing
phenomena, where galaxies with currently larger stellar masses previously had
higher AGN energetic contributions and star-formation-originating infrared
luminosities, and have finished their major star-formation more quickly, due to
stronger AGN feedback.
