Comprehensive Search for Velocity Shifts of Broad Absorption Lines in Quasars with Multi-Epoch SDSS Observations

Supermassive black holes have a profound impact on the gas in galaxies, releasing energy in the form of radiation and kinetic energy that can significantly influence the galaxy's evolution. This phenomenon, known as AGN feedback, plays a crucial role in the formation and evolution of host galaxies (Di Matteo et al. 2005; Hopkins et al. 2008; Fabian 2012). Quasars generate outflows that are an important form of feedback, producing blueshifted broad absorption line features (BALs) that are defined as absorption troughs that are at least 2000 km s-1 wide (Weymann et al. 1991). By studying the variability and velocity shift of BALs, researchers can gain insight into the lifetimes, locations, and geometries of quasar winds and provide constraints on dynamical outflow models (Filiz Ak et al. 2013; Silk & Rees 1998; Fabian et al. 2010; Di Matteo et al. 2005).

However, the physical mechanism that causes the velocity shift of BALs is still controversial, and the models proposed have not been supported by strong observational evidence. Limited research on the physical mechanism is due to the scarcity of observation cases. Although some studies have reported velocity shifts of BALs in individual sources, they are rare in quasar outflows (Vilkoviskij & Irwin 2001; Rupke et al. 2002; Hall et al. 2007; Joshi et al. 2014; Grier et al. 2016). Recent studies have reported on a few cases of BALs with velocity shifts (Lu & Lin 2019; Yao et al. 2020; Lu & Lin 2020; Lin & Lu 2020), suggesting that outflows in these sources are affected by radiation from the central source. However, further research is limited by the scarcity of cases.

To gain a deeper understanding of outflowing absorption line velocity shifts, this study aims to comprehensively search for BAL velocity shifts from quasars with multi-epoch observations by the Sloan Digital Sky Survey (SDSS) and perform statistical analysis based on the obtained sample. The article outlines the sample selection, methods used for spectral analysis, identification of BALs, and measurement of relevant properties. The detected BALs in the 25 quasar sample are described, and statistical analysis results and discussions are presented. The study aims to contribute to a better understanding of the physical mechanism that causes the velocity shift of BALs.