2018MNRAS.480.3201K

We study the narrow emission-line properties and stellar populations of a sample of 1385 active galactic nuclei (AGNs) selected to have strong excess emission at mid-infrared (mid-IR) 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 AGNs selected by their optical emission-line ratios. To enable this comparison, we construct a control sample of 50 000 optically selected AGNs with the same redshifts that do not have strong mid-IR excess emission. The mid-IR excess and control samples differ strongly in [O III] line luminosity, ionized gas excitation mechanism, ionization state, and electron density. We show that the radio-detected, mid-IR excess AGNs constitute the most luminous and highly ionized AGNs 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 AGNs 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 constitutes a class of lower ionization, less luminous AGNs in more quiescent galaxies that contribute primarily to the growth of low mass black holes.