Understanding Velocity Shifts in BAL Quasars: A Scarcity of Observation Cases

The observed velocity shift of BALs can be explained by several possible causes: (1) the actual acceleration or deceleration of the outflowing material in the direction of the observer's line of sight, (2) the outflowing material passing through the observer's line of sight when moving in a circular motion around the central massive black hole, and (3) changes in the physical properties of the outflowing material, such as ionization state and column density. However, the above three explanations and the models they rely on have not been supported by strong observational evidence. In other words, the physical mechanism that causes the velocity shift of BALs is still controversial.

Research on the physical mechanism that causes the velocity shift of BALs is limited due to the scarcity of observation cases. About two decades ago, the first report of the BAL velocity shift was presented by Vilkoviskij & Irwin (2001). Until recent years, only a few cases of quasars with BALs and velocity shifts have been discovered (e.g., Rupke et al. 2002; Hall et al. 2007; Joshi et al. 2014; Grier et al. 2016; Joshi et al. 2019; Lu & Lin 2019; Xu et al. 2020; Yao et al. 2020; Lu & Lin 2020a,b; Lin & Lu 2020). In all of these studies that detected the velocity shift of BALs, only Grier et al. (2016) searched for velocity shifts from a quasar sample, not from individual sources. Nevertheless, in the sample containing 140 BAL quasar sources, Grier et al. (2016) only found three cases of velocity shifts. One of the reasons why Grier et al. (2016) can only identify so few cases of C IV BALs showing velocity shifts is that their criteria for researching the BAL with velocity shift are quite strict.