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 mu m (L-band) spectroscopy of nearby infrared luminous galaxies with possible signatures of dust-obscured active galactic nuclei (AGNs) in data at other wavelengths. The samples are chosen to include sources with a radio excess relative to far-infrared emission, strong absorption features in mid-infrared 5-11.5 mu m spectra, unusually weak [C-II] 158 mu m emission relative to the far-infrared continuum, and radio galaxies classified optically as narrow-line objects. Our aim is to investigate whether the signatures of possible obscured AGNs can be detected in our L-band spectra based on the strengths of emission and absorption features. Six of nine observed sources clearly show 3.3 mu m polycyclic aromatic hydrocarbon emission features, a good starburst indicator. An absorption feature at 3.1 mu m due to ice-covered dust is detected in IRAS 04154+1755 and IRAS 17208-0014. The signature of a bare carbonaceous dust absorption feature at 3.4 mu m is seen in NGC 1377. Our L-band spectra reveal strong signatures of obscured AGNs in all three optical Seyfert 2 galaxies ( IRAS 04154+1755, Cygnus A, and 3C 234) and two galaxies classified optically as non-Seyfert galaxies ( NGC 828 and NGC 1377). Among the remaining optical non-Seyfert galaxies, IRAS 17208-0014 might also show a buried AGN signature, whereas no explicit AGN evidence is seen in the L-band spectra of the mid-infrared absorption feature source IRAS 15250+3609 and two weak [C-II] emitters, IC 860 and CGCG 1510.8+0725.