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 IRS low-resolution infrared 5-35 micronspectroscopy of nearby ultraluminous infrared galaxies (ULIRGs) at z < 0.15. Wefocus on the search for the signatures of buried active galactic nuclei (AGNs)in the complete sample of ULIRGs classified optically as non-Seyferts (LINERsor HII-regions). In addition to polycyclic aromatic hydrocarbon (PAH) emissionfeatures at 6.2 micron, 7.7 micron, and 11.3 micron, the conventional tool ofstarburst-AGN separation, we use the optical depths of the 9.7 micron and 18micron silicate dust absorption features to infer the geometry of energysources and dust at the nuclei of these ULIRGs, namely, whether the energysources are spatially well mixed with dust(a normal starburst) or are morecentrally concentrated than the dust (a buried AGN). Infrared spectra of atleast 30%, and possibly 50%, of the observed optical non-Seyfert ULIRGs arenaturally 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 energysources with no PAH emission. We interpret the latter component as a buriedAGN. The fraction of ULIRGs showing some buried AGN signatures is higher inLINER ULIRGs than in HII-region ULIRGs. Most of the luminous buried AGNcandidates are found in ULIRGs with cool far-infrared colors. Where theabsorption-corrected intrinsic AGN luminosities are derivable with littleuncertainty, they are found to be of the order of 10^12Lsun, accounting for thebulk of the ULIRGs' luminosities. The 5-35 micron spectroscopic starburst/AGNclassifications are generally consistent with our previous classificationsbased on 3-4 micron spectra for the same sample.Comment: 67 pages, 13 figures, accepted for publication in Ap