Are Broad Mg II Galaxies Fading AGN?

Abstract

We analyzed a sample of 494 galaxies from the Sloan Digital Sky Survey (SDSS) I-IV that have unusual properties. They display broadened Mg II $λ$2798 line emission, as would be expected of Type 1 active galactic nuclei (AGN). However, they have weak or absent broad Balmer lines though broadened Balmer lines often accompany broadened ultraviolet lines in Type 1 AGN spectra. We performed a spectral decomposition of these "broad Mg II galaxies'' to assess whether they can be considered fading AGN. The Balmer lines are more responsive to changes in the AGN continuum than the Mg II line. As the AGN power source fades, broad H$β$ and then H$α$ will weaken and eventually disappear, leaving broad Mg II as a signpost of a more active past. In support of this interpretation, we found that broad Mg II galaxies have weaker AGN luminosities and lower accretion rates (parameterized by the Eddington ratio) than typical Type 1 quasars from SDSS. The Eddington ratio of broad Mg II galaxies are similar to dim-state changing-look AGN, with an average value $\lesssim$1\%, which is the threshold at which the inner region of a geometrically thin accretion disk is thought to evaporate and be replaced by a geometrically thick radiatively inefficient accretion flow in stellar-mass black holes. Broad Mg II galaxies may represent the penultimate stage in a changing-look AGN sequence prior to the complete disappearance of all broad lines.