Multi-epoch spectroscopy of Mg II broad absorption line transitions

Abstract

Built upon a sample of 134 quasars that was dedicated to a systematic study of Mg ii-BAL variability from Yi et al. (2019a), we investigate these quasars showing Mg ii-BAL disappearance or emergence with the aid of at least three epoch optical spectra sampled more than 15 yr in the observed frame. We identified 3/3 quasars undergoing pristine/tentative BAL transformations. The incidence of pristine BAL transformations in the sample is therefore derived to be 2.2 +2.2 −1.2 %, consistent with that of highionization BAL transformations from the literature. Adopting an average Mg ii-BAL disappearance timescale of rest-frame 6.89 yr among the six quasars, the average characteristic lifetime of Mg ii BALs in the sample is constrained to be >160 yr along our line of sight. There is a diversity of BAL-profile variability observed in the six quasars, probably reflecting a variety of mechanisms at work. Our investigations of Mg ii-BAL transitions, combined with observational studies of BAL transitions from the literature, imply an overall FeLoBAL/LoBAL→HiBAL/non-BAL transformation sequence along with a decrease in reddening. This sequence is consistent with the evacuation models for the origin of commonly seen blue quasars, in which LoBAL quasars are in a shorted-lived, blowout phase.