AKARI Near-Infrared Spectroscopy of Luminous Infrared Galaxies

TL;DR

This study uses AKARI near-infrared spectroscopy to identify buried AGN in luminous infrared galaxies, revealing that many optically non-Seyfert galaxies host hidden AGN contributing modestly to their infrared luminosity.

Contribution

It demonstrates that NIR spectral features can uncover buried AGN in (U)LIRGs and correlates NIR continuum slopes with WISE colors for efficient AGN identification.

Findings

Half of optically non-Seyfert (U)LIRGs show AGN signatures in NIR spectra.

Buried AGN contribute 5-10% to the infrared luminosity.

NIR continuum slopes correlate with WISE [3.4]-[4.6] colors.

Abstract

We present the AKARI near-infrared (NIR; 2.5-5 micron) spectroscopic study of 36 (ultra)luminous infrared galaxies [(U)LIRGs] at z=0.01-0.4. We measure the NIR spectral features including the strengths of 3.3 micron polycyclic aromatic hydrocarbon (PAH) emission and hydrogen recombination lines (Br\alpha. and Br\beta), optical depths at 3.1 and 3.4 micron, and NIR continuum slope. These spectral features are used to identify optically elusive, buried AGN. We find that half of the (U)LIRGs optically classified as non-Seyferts show AGN signatures in their NIR spectra. Using a combined sample of (U)LIRGs with NIR spectra in the literature, we measure the contribution of buried AGN to the infrared luminosity from the SED-fitting to the IRAS photometry. The contribution of these buried AGN to the infrared luminosity is 5-10%, smaller than the typical AGN contribution of (U)LIRGs including Seyfert galaxies (10-40%). We show that NIR continuum slopes correlate well with WISE [3.4]-[4.6] colors, which would be useful for identifying a large number of buried AGN using the WISE data.