SPITZER INFRARED LOW-RESOLUTION SPECTROSCOPIC STUDY OF BURIED ACTIVE GALACTIC NUCLEI IN A COMPLETE SAMPLE OF NEARBY ULTRALUMINOUS INFRARED GALAXIES

We present the results of Spitzer Infrared Spectrograph low-resolution infrared 5-35 mu m spectroscopy of 17 nearby ultraluminous infrared galaxies (ULIRGs) at z < 0.2, optically classified as non-Seyferts. The presence of optically elusive, but intrinsically luminous, buried active galactic nuclei (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 similar to 10(12) L-circle dot, 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.