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

TL;DR

This study uses AKARI IRC spectroscopy to analyze 22 infrared galaxies, revealing that buried AGNs are common and their activity increases with luminosity, while star formation remains the dominant energy source.

Contribution

It provides the first systematic infrared spectroscopic analysis across a wide luminosity range, highlighting the prevalence and increasing contribution of buried AGNs in luminous infrared galaxies.

Findings

Buried AGNs are prevalent in U/LIRGs.

AGN activity increases with infrared luminosity.

Star formation dominates the infrared luminosity.

Abstract

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.