Infrared 3-4 μm Spectroscopy of Infrared Luminous Galaxies with Possible Signatures of Obscured Active Galactic Nuclei

We present the results of infrared 2.8-4.1 micron (L-band) spectroscopy ofnearby infrared luminous galaxies with possible signatures of dust-obscuredactive galactic nuclei (AGNs) in data at other wavelengths. The samples arechosen to include sources with a radio excess relative to far-infraredemission, strong absorption features in mid-infrared 5-11.5 micron spectra,unusually weak [CII] 158 micron emission relative to the far-infraredcontinuum, and radio galaxies classified optically as narrow-line objects. Ouraim is to investigate whether the signatures of possible obscured AGNs can bedetected in our L-band spectra, based on the strengths of emission andabsorption features. Six of nine observed sources clearly show 3.3 micronpolycyclic aromatic hydrocarbon (PAH) emission features, a good starburstindicator. An absorption feature at 3.1 micron due to ice-covered dust isdetected in IRAS 04154+1755 and IRAS 17208-0014. The signature of a barecarbonaceous dust absorption feature at 3.4 micron is seen in NGC 1377. OurL-band spectra reveal strong signatures of obscured AGNs in all three opticalSeyfert 2 galaxies (IRAS 04154+1755, Cygnus A, and 3C 234), and two galaxiesclassified optically as non-Seyferts (NGC 828 and NGC 1377). Among theremaining optical non-Seyferts, IRAS 17208-0014 might also show a buried AGNsignature, whereas no explicit AGN evidence is seen in the L-band spectra ofthe mid-infrared absorption-feature source IRAS 15250+3609, and two weak [CII]emitters IC 860 and CGCG 1510.8+0725.Comment: 14 pages (emulateapj.sty), 3 figures, Accepted for publication in Astronomical Journal Vol. 131 No. 5 (May 2006