Towards a Complete Census of AGNs in Nearby Galaxies: A Large Population of Optically Unidentified AGNs

TL;DR

This study uses infrared spectroscopy to reveal a significant population of AGNs in nearby galaxies that are missed by optical surveys, highlighting the importance of IR diagnostics for complete AGN census.

Contribution

It provides the first large-scale infrared-based census showing many optically unidentified AGNs, emphasizing the role of host galaxy extinction in AGN detection.

Findings

27% of IR-bright galaxies host AGNs, over half are optically unidentified.

Optically unidentified AGNs are often in dusty, inclined galaxies.

Obscuration by host galaxy dust, not just torus, causes optical misclassification.

Abstract

Using Spitzer-IRS spectroscopy, we investigate the ubiquity of Active Galactic Nuclei (AGN) in a complete (~94%), volume-limited sample of the most bolometrically-luminous galaxies (L_IR > (0.3-20) x 10^10 L_sun) to D < 15 Mpc. Our analyses are based on the detection of the high-excitation emission line [NeV](\lambda 14.32 um; 97.1 eV) to unambiguously identify AGN activity. We find that 17 of the 64 IR-bright galaxies in our sample host AGN activity (~27^{+8}_{-6}%), >50% of which are not identified as AGNs using optical spectroscopy. The large AGN fraction indicates a tighter connection between AGN activity and IR luminosity for galaxies in the local Universe than previously found, potentially indicating a close association between AGN activity and star formation. The optically unidentified AGNs span a wide range of galaxy type (S0-Ir) and are typically starburst-dominated systems hosting modest-luminosity AGN activity (L_[NeV] ~ 10^37 - 10^39 erg s^-1). The non-identification of optical AGN signatures in the majority of these galaxies appears to be due to extinction towards the AGN, rather than intrinsically low-luminosity AGN activity. Examination of optical images shows that the optically unidentified AGNs with evidence for extinction are hosted in either highly inclined galaxies or galaxies with dust lanes, indicating that obscuration of the AGN is not necessarily due to an obscuring torus. We therefore conclude that optical spectroscopic surveys miss approximately half of the AGN population simply due to extinction through the host galaxy.