Properties of AGN selected by their mid-IR colours: evidence for a physically distinct mode of black hole growth

TL;DR

This study compares mid-infrared selected AGN with optically-selected AGN to reveal distinct physical conditions and growth modes of black holes, highlighting the role of galaxy interactions and starbursts.

Contribution

It demonstrates that mid-IR excess AGN are more luminous, highly ionized, and linked to galaxy mergers, representing a distinct black hole growth mode compared to optically-selected AGN.

Findings

Mid-IR excess AGN are the most luminous and highly ionized in the local universe.

They primarily grow black holes in massive, starburst galaxies triggered by mergers.

Control AGN are less luminous, lower ionization, and in quiescent galaxies.

Abstract

We study the narrow emission line properties and stellar populations of a sample of 1385 AGN selected to have strong excess emission at mid-infrared wavelengths based on comparing Wide-field Infrared Survey Explorer W1-W2 band colours with optical stellar absorption line indicators. Our goal is to understand whether the physical conditions in the interstellar medium of these objects differ from those of local AGN selected by their optical emission line ratios. To enable this comparison, we construct a control sample of 50,000 optically-selected AGN with the same redshifts that do not have strong mid-IR excess emission. The mid-IR excess and control samples differ strongly in [OIII] line luminosity, ionized gas excitation mechanism, ionization state and electron density. We show that the radio-detected, mid-IR excess AGN constitute the most luminous and highly ionized AGN in the local Universe and they contribute primarily to the growth of black holes in the most massive galaxies. At least half of this black hole growth is occurring in galaxies with recent starbursts. The morphologies of these systems indicate that the starbursts have probably been triggered by galaxy-galaxy mergers and interactions. The most luminous AGN in our mid-IR excess sample have properties that are similar to the Type II quasars studied at higher redshifts. In contrast, the control sample constitute a class of lower ionization, less luminous AGN in more quiescent galaxies that contribute primarily to the growth of low mass black holes.