Infrared 3–4 μm Spectroscopic Investigations of a Large Sample of Nearby Ultraluminous Infrared Galaxies

We present infrared L-band (3-4 micron) nuclear spectra of a large sample ofnearby ultraluminous infrared galaxies (ULIRGs).ULIRGs classified optically asnon-Seyferts (LINERs, HII-regions, and unclassified) are our main targets.Using the 3.3 micron polycyclic aromatic hydrocarbon (PAH) emission andabsorption features at 3.1 micron due to ice-covered dust and at 3.4 micronproduced by bare carbonaceous dust, we search for signatures of powerful activegalactic nuclei (AGNs) deeply buried along virtually all lines-of-sight. The3.3 micron PAH emission, the signatures of starbursts, is detected in all buttwo non-Seyfert ULIRGs, but the estimated starburst magnitudes can account foronly a small fraction of the infrared luminosities. Three LINER ULIRGs showspectra typical of almost pure buried AGNs, namely, strong absorption featureswith very small equivalent-width PAH emission. Besides these three sources, 14LINER and 3 HII ULIRGs' nuclei show strong absorption features whose absoluteoptical depths suggest an energy source more centrally concentrated than thesurrounding dust, such as a buried AGN. In total, 17 out of 27 (63%) LINER and3 out of 13 (23%) HII ULIRGs' nuclei show some degree of evidence for powerfulburied AGNs, suggesting that powerful buried AGNs may be more common in LINERULIRGs than in HII ULIRGs. The evidence of AGNs is found in non-Seyfert ULIRGswith both warm and cool far-infrared colors. These spectra are compared withthose of 15 ULIRGs' nuclei with optical Seyfert signatures taken forcomparison.The overall spectral properties suggest that the total amount ofdust around buried AGNs in non-Seyfert ULIRGs is systematically larger thanthat around AGNs in Seyfert 2 ULIRGs.Comment: 56 pages, 9 figures, accepted for publication in ApJ (20 January 2006, vol 637 issue