Multi-epoch spectroscopy of Mg II broad absorption line transitions

TL;DR

This study uses multi-epoch spectroscopy over 15 years to analyze Mg II broad absorption line variability in quasars, revealing low transformation rates and suggesting a short-lived, dynamic BAL phase in quasar evolution.

Contribution

It provides the first long-term observational constraints on Mg II BAL transformation rates and lifetimes, supporting models of short-lived BAL phases in quasar evolution.

Findings

Pristine Mg II BAL transformation rate is ~2.2%.

Average Mg II BAL lifetime exceeds 160 years.

Diversity in BAL-profile variability indicates multiple mechanisms.

Abstract

Built upon a sample of 134 quasars that was dedicated to a systematic study of \mgii-BAL variability from Yi et al. (2019a), we investigate these quasars showing \mgii-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$_{-1.2}^{+2.2}$\%, consistent with that of high-ionization BAL transformations from the literature. Adopting an average \mgii-BAL disappearance timescale of rest-frame 6.89 yr among the six quasars, the average characteristic lifetime of \mgii\ BALs in the sample is constrained to be $>$160 yr along our line of sight. There is a diversity of BAL-profile variability observed in the six quasars, probably reflecting a variety of mechanisms at work. Our investigations of \mgii-BAL transitions, combined with observational studies of BAL transitions from the literature, imply an overall FeLoBAL/LoBAL$\rightarrow$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.