The Associated Absorption Features in Quasar Spectra of the Sloan Digital Sky Survey. I. Mg ii Absorption Doublets

Using the SDSS spectra of quasars included in the DR7Q or DR12Q catalogs, we search for Mg ii λλ2796, 2803 narrow absorption doublets in the spectra data around Mg ii λ2798 emission lines. We obtain 17,316 Mg ii doublets, within the redshift range of 0.3299 ≤ zabs ≤ 2.5663. We find that a velocity offset of r < 6000 km s−1 is a safe boundary to constrain the vast majority of associated Mg ii systems, although we find some doublets at r > 6000 km s−1. If associated Mg ii absorbers are defined by r < 6000 km s−1, ∼33.3% of the absorbers are supposed to be contaminants of intervening systems. Removing the 33.3% contaminants, ∼4.5% of the quasars present at least one associated Mg ii system with . The fraction of associated Mg ii systems with high-velocity outflows correlates with the average luminosities of their central quasars, indicating a relationship between outflows and the quasar feedback power. The r distribution of the outflow Mg ii absorbers is peaked at 1023 km s−1, which is smaller than the corresponding value of the outflow C iv absorbers. The redshift number density evolution of absorbers (dn/dz) limited by r > −3000 km s−1 differs from that of absorbers constrained by r > 2000 km s−1. Absorbers limited by r > 2000 km s−1 and higher values exhibit profiles similar to dn/dz. In addition, the dn/dz is smaller when absorbers are constrained with larger r. The distributions of equivalent widths, and the ratio of , are the same for associated and intervening systems, and independent of quasar luminosity.