AKARI NEAR-INFRARED SPECTROSCOPY OF LUMINOUS INFRARED GALAXIES

We present the AKARI near-infrared (NIR; 2.5-5 mu m) spectroscopic study of 36 (ultra) luminous infrared galaxies ((U) LIRGs) at z = 0.01-0.4. We measure the NIR spectral features including the strengths of 3.3 mu m polycyclic aromatic hydrocarbon emission and hydrogen recombination lines (Br alpha and Br beta), optical depths at 3.1 and 3.4 mu m, and NIR continuum slope. These spectral features are used to identify optically elusive, buried active galactic nuclei (AGNs). We find that half of the (U) LIRGs optically classified as non-Seyferts show AGN signatures in their NIR spectra. Using a combined sample of (U) LIRGs with NIR spectra in the literature, we measure the contribution of buried AGNs to the infrared luminosity from the spectral energy distribution fitting to the IRAS photometry. The contribution of these buried AGNs to the infrared luminosity is 5%-10%, smaller than the typical AGN contribution of (U) LIRGs including Seyfert galaxies (10%-40%). We show that NIR continuum slopes correlate well with WISE [3.4]-[4.6] colors, which would be useful for identifying a large number of buried AGNs using the WISE data.