<i>AKARI</i>IRC INFRARED 2.5-5 μm SPECTROSCOPY OF A LARGE SAMPLE OF LUMINOUS INFRARED GALAXIES

Abstract

We present the results of our systematic infrared 2.5-5 micron spectroscopy\nof 60 luminous infrared galaxies (LIRGs) with infrared luminosities L(IR) =\n10^11-12 Lsun, and 54 ultraluminous infrared galaxies (ULIRGs) with L(IR) &gt;\n10^12 Lsun, using AKARI IRC. AKARI IRC slit-less spectroscopy allows us to\nprobe the full range of emission from these galaxies, including spatially\nextended components. The 3.3 micron polycyclic aromatic hydrocarbon (PAH)\nemission features, hydrogen recombination emission lines, and various\nabsorption features are detected and used to investigate the properties of\nthese galaxies. Because of the relatively small effect of dust extinction in\nthe infrared range, quantitative discussion of these dusty galaxy populations\nis possible. For sources with clearly detectable Br beta (2.63 micron) and Br\nalpha (4.05 micron) emission lines, the flux ratios are found to be similar to\nthat predicted by case B theory. Starburst luminosities are estimated from both\n3.3 micron PAH and Br alpha emission, which roughly agree with each other. In\naddition to the detected starburst activity, a significant fraction of the\nobserved sources display signatures of obscured AGNs, such as low PAH\nequivalent widths, large optical depths of dust absorption features, and red\ncontinuum emission. The energetic importance of optically elusive buried AGNs\nin optically non-Seyfert galaxies tends to increase with increasing galaxy\ninfrared luminosity, from LIRGs to ULIRGs.\n