Spitzer Infrared Low-Resolution Spectroscopic Study of Buried AGNs in a
  Complete Sample of Nearby Ultraluminous Infrared Galaxies
  We present the results of Spitzer IRS low-resolution infrared 5-35 micron
spectroscopy of 17 nearby ULIRGs at z < 0.2, optically classified as
non-Seyferts. The presence of optically elusive, but intrinsically luminous,
buried AGNs is investigated, based on the strengths of polycyclic aromatic
hydrocarbon emission and silicate dust absorption features detected in the
spectra. The signatures of luminous buried AGNs, whose intrinsic luminosities
range up to ~10^12 Lsun, are found in eight sources. We combine these results
with those of our previous research to investigate the energy function of
buried AGNs in a complete sample of optically non-Seyfert ULIRGs in the local
universe at z < 0.3 (85 sources). We confirm a trend that we previously
discovered: that buried AGNs are more common in galaxies with higher infrared
luminosities. Because optical Seyferts also show a similar trend, we argue more
generally that the energetic importance of AGNs is intrinsically higher in more
luminous galaxies, suggesting that the AGN-starburst connections are
luminosity-dependent. This may be related to the stronger AGN feedback scenario
in currently more massive galaxy systems, as a possible origin of the galaxy
downsizing phenomenon.
