Luminous buried AGNs as a function of galaxy infrared luminosity revealed through Spitzer low-resolution infrared spectroscopy

TL;DR

This study uses Spitzer infrared spectroscopy to reveal that buried active galactic nuclei (AGNs) become increasingly important in powering the infrared luminosity of ultraluminous infrared galaxies (ULIRGs) as their luminosity exceeds 10^12 solar luminosities, supporting galaxy evolution models involving AGN feedback.

Contribution

It provides the first systematic evidence that buried AGNs dominate the energy output in the most luminous ULIRGs, linking AGN activity with galaxy mass and evolution.

Findings

Buried AGNs become prominent in ULIRGs with L(IR) > 10^12 Lsun.

A significant fraction of infrared luminosity in these ULIRGs is due to buried AGNs.

Results support the AGN-feedback scenario in galaxy downsizing.

Abstract

We present the results of Spitzer IRS infrared 5-35 micron low-resolution spectroscopic energy diagnostics of ultraluminous infrared galaxies (ULIRGs) at z > 0.15, classified optically as non-Seyferts. Based on the equivalent widths of polycyclic aromatic hydrocarbon emission and the optical depths of silicate dust absorption features, we searched for signatures of intrinsically luminous, but optically elusive, buried AGNs in these optically non-Seyfert ULIRGs. We then combined the results with those of non-Seyfert ULIRGs at z < 0.15 and non-Seyfert galaxies with infrared luminosities L(IR) < 10^12Lsun. We found that the energetic importance of buried AGNs clearly increases with galaxy infrared luminosity, becoming suddenly discernible in ULIRGs with L(IR) > 10{12}Lsun. For ULIRGs with buried AGN signatures, a significant fraction of infrared luminosities can be accounted for by detected buried AGN and modestly-obscured (Av < 20 mag) starburst activity. The implied masses of spheroidal stellar components in galaxies for which buried AGNs become important roughly correspond to the value separating red massive and blue, less-massive galaxies in the local universe. Our results may support the widely-proposed AGN-feedback scenario as the origin of galaxy downsizing phenomena, where galaxies with currently larger stellar masses previously had higher AGN energetic contributions and star-formation-originating infrared luminosities, and have finished their major star-formation more quickly, due to stronger AGN feedback.