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We present the result of a systematic infrared 2.5-5 um spectroscopic studyof 22 nearby infrared galaxies over a wide infrared luminosity range (10 <log(L_IR / Lsun) < 13) obtained from AKARI Infrared Camera (IRC). The uniqueband of the AKARI IRC spectroscopy enables us to access both the 3.3 umpolycyclic aromatic hydrocarbon (PAH) emission feature from star formingactivity and the continuum of torus-dust emission heated by an active galacticnucleus (AGN). Applying our AGN diagnostics to the AKARI spectra, we discover14 buried AGNs. The large fraction of buried AGNs suggests that AGN activitybehind the dust is almost ubiquitous in ultra-/luminous infrared galaxies(U/LIRGs). We also find that both the fraction and energy contribution ofburied AGNs increase with infrared luminosity from 10 < log(L_IR / Lsun) < 13,including normal infrared galaxies with log (L_IR / Lsun) < 11. The energycontribution from AGNs in the total infrared luminosity is only ~7% in LIRGsand ~20% in ULIRGs, suggesting that the majority of the infrared luminosityoriginates from starburst activity. Using the PAH emission, we investigate theluminosity relation between star formation and AGN. We find that these infraredgalaxies exhibit higher star formation rates than optically selected Seyfertgalaxies with the same AGN luminosities, implying that infrared galaxies couldbe an early evolutionary phase of AGN.

AKARI IRC 2.5-5 um Spectroscopy of Infrared Galaxies over a Wide Luminosity Range