Discovery of an Mg II Changing-look Active Galactic Nucleus and Its Implications for a Unification Sequence of Changing-look Active Galactic Nuclei

Changing look (CL) is a rare phenomenon of active galactic nuclei (AGNs) that exhibit emerging or disappearing broad lines accompanied by continuum variations on astrophysically short timescales (≲1 yr to a few decades). While previous studies have found Balmer-line (broad Hα and/or Hβ) CL AGNs, the broad Mg ii line is persistent even in dim states. No unambiguous Mg ii CL AGN has been reported to date. We perform a systematic search of Mg ii CL AGNs using multi-epoch spectra of a special population of Mg ii-emitters (characterized by strong broad Mg ii emission with little evidence for AGNs from other normal indicators such as broad Hα and Hβ or blue power-law continua) from the Sloan Digital Sky Survey Data Release 14. We present the discovery of the first unambiguous case of a Mg ii CL AGN, SDSS J152533.60+292012.1 (at redshift z = 0.449), which is turning off within rest-frame 286 days. The dramatic diminishing of Mg ii equivalent width (from 110 ± 26 Å to being consistent with zero), together with little optical continuum variation (ΔV _max-min = 0.17 ± 0.05 mag) coevally over ∼10 yr, rules out dust extinction or a tidal disruption event. Combined with previously known Hβ CL AGNs, we construct a sequence that represents different temporal stages of CL AGNs. This CL sequence is best explained by the photoionization model of Guo et al. In addition, we present two candidate turn-on Mg ii CL AGNs and a sample of 361 Mg ii-emitters for future Mg ii CL AGN searches.