Unraveling the Mystery of BAL Velocity Shifts in Quasars: A Call for More Observations

The observed velocity shift of BALs are 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 passes through the observer's line of sight when moving in a circular motion around the central massive black hole, (3) changes in the physical properties of the outflowing material, such as ionization state and column density etc. 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.

The research on the physical mechanism that causes the velocity shift of BAL is limited by the scarcity of observation cases. About two decades before, the first report of the BAL velocity shift was presented by Vilkoviskij & Irwin (2001). Until recent years, only a few quasar cases of BALs with 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 shift from a quasar sample, not from individual source. Nevertheless, in the sample containing 140 BAL quasar sources, Grier et al. (2016) only found three cases of velocity shifts. Of course, 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.

However, the limited number of observation cases does not necessarily mean that the physical mechanism causing the velocity shift of BALs is rare or uncommon. It is possible that many BALs with velocity shifts have not been detected yet due to the limitations of current observational technology and methods. Therefore, more systematic and comprehensive studies are needed to explore the physical mechanism of BAL velocity shifts and to understand the properties and evolution of outflows from quasars and active galactic nuclei.