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