Spitzer Infrared Low-Resolution Spectroscopic Study of Buried AGNs in a Complete Sample of Nearby Ultraluminous Infrared Galaxies

TL;DR

This study uses Spitzer infrared spectroscopy to identify buried active galactic nuclei (AGNs) in nearby ultraluminous infrared galaxies, revealing their prevalence and potential role in galaxy evolution.

Contribution

It provides the first comprehensive analysis of buried AGNs in a complete sample of optically non-Seyfert ULIRGs, linking AGN activity to galaxy luminosity and evolution.

Findings

Buried AGNs are present in 8 out of 17 studied ULIRGs.

Higher infrared luminosity correlates with increased buried AGN occurrence.

The energetic significance of AGNs increases with galaxy luminosity.

Abstract

We present the results of Spitzer IRS low-resolution infrared 5-35 micron spectroscopy of 17 nearby ULIRGs at z < 0.2, optically classified as non-Seyferts. The presence of optically elusive, but intrinsically luminous, buried AGNs is investigated, based on the strengths of polycyclic aromatic hydrocarbon emission and silicate dust absorption features detected in the spectra. The signatures of luminous buried AGNs, whose intrinsic luminosities range up to ~10^12 Lsun, are found in eight sources. We combine these results with those of our previous research to investigate the energy function of buried AGNs in a complete sample of optically non-Seyfert ULIRGs in the local universe at z < 0.3 (85 sources). We confirm a trend that we previously discovered: that buried AGNs are more common in galaxies with higher infrared luminosities. Because optical Seyferts also show a similar trend, we argue more generally that the energetic importance of AGNs is intrinsically higher in more luminous galaxies, suggesting that the AGN-starburst connections are luminosity-dependent. This may be related to the stronger AGN feedback scenario in currently more massive galaxy systems, as a possible origin of the galaxy downsizing phenomenon.