A Spitzer IRS low-resolution spectroscopic search for buried AGNs in nearby ultraluminous infrared galaxies: A constraint on geometry between energy sources and dust

We present the results of Spitzer Infrared Spectrograph low-resolution infrared 5-35 μm spectroscopy of nearby ultraluminous infrared galaxies (ULIRGs) at z<0.15. We focus on the search for the signatures of buried active galactic nuclei (AGNs) in the complete sample of ULIRGs classified optically as non-Seyferts (LINERs or H II regions). In addition to polycyclic aromatic hydrocarbon (PAH) emission features at 6.2, 7.7, and 11.3 μm, the conventional tool of starburst-AGN separation, we use the optical depths of the 9.7 and 18 μm silicate dust absorption features to infer the geometry of energy sources and dust at the nuclei of these ULIRGs, namely, whether the energy sources are spatially well mixed with dust (a normal starburst) or are more centrally concentrated than the dust (a buried AGN). Infrared spectra of at least 30%, and possibly 50%, of the observed optical non-Seyfert ULIRGs are naturally explained by emission consisting of (1) energetically insignificant, modestly obscured (AV<20-30 mag) PAH-emitting normal starbursts and (2) energetically dominant, highly dust-obscured, centrally concentrated energy sources with no PAH emission. We interpret the latter component as a buried AGN. The fraction of ULIRGs showing some buried AGN signatures is higher in LINER ULIRGs than in H II region ULIRGs. Most of the luminous buried AGN candidates are found in ULIRGs with cool far-infrared colors. Where the absorption-corrected intrinsic AGN luminosities are derivable with little uncertainty, they are found to be of the order of 1012 Lsolar, accounting for the bulk of the ULIRGs' luminosities. The 5-35 μm spectroscopic starburst/AGN classifications are generally consistent with our previous classifications based on 3-4 μm spectra for the same sample.