We present the results of our systematic infrared 2.5-5 micron spectroscopy of 60 luminous infrared galaxies (LIRGs) with infrared luminosities L(IR) = 10^11-12 Lsun, and 54 ultraluminous infrared galaxies (ULIRGs) with L(IR) > 10^12 Lsun, using AKARI IRC. AKARI IRC slit-less spectroscopy allows us to probe the full range of emission from these galaxies, including spatially extended components. The 3.3 micron polycyclic aromatic hydrocarbon (PAH) emission features, hydrogen recombination emission lines, and various absorption features are detected and used to investigate the properties of these galaxies. Because of the relatively small effect of dust extinction in the infrared range, quantitative discussion of these dusty galaxy populations is possible. For sources with clearly detectable Br beta (2.63 micron) and Br alpha (4.05 micron) emission lines, the flux ratios are found to be similar to that predicted by case B theory. Starburst luminosities are estimated from both 3.3 micron PAH and Br alpha emission, which roughly agree with each other. In addition to the detected starburst activity, a significant fraction of the observed sources display signatures of obscured AGNs, such as low PAH equivalent widths, large optical depths of dust absorption features, and red continuum emission. The energetic importance of optically elusive buried AGNs in optically non-Seyfert galaxies tends to increase with increasing galaxy infrared luminosity, from LIRGs to ULIRGs.
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