Buried AGNs in nearby ultraluminous infrared galaxies

We present infrared L-band (3-4 mu m) nuclear spectra of a large sample of nearby ultraluminous infrared galaxies (ULIRGs). Using the 3.3 mu m polycyclic aromatic hydrocarbon (PAH) emission and absorption features found at 3-4 mu m, we search for signatures of optically elusive powerful active galactic nuclei (AGNs) deeply buried in dust in all directions. Among ULIRGs classified optically as "non-Seyfert", the signatures of powerful buried AGNs are found in a much higher fraction of optical LINER ULIRGs than in HII-region ULIRGs. The evidence of AGNs is found in optical non-Seyfert ULIRGs with both warm and cool far-infrared colors. The overall spectral properties suggest that the total amount of dust around buried AGNs in optical non-Seyfert ULIRGs is systematically larger than that around AGNs in optical Seyfert 2 ULIRGs.