Collective properties of quasar narrow associated absorption lines

Abstract: This paper statistically investigates the properties of C IV and Mg II narrow absorption lines (NALs) to look for velocity cuts that can well constrain quasar-associated NALs. The coverage fraction (f_c) is defined as the ratio between the number of quasars exhibiting at least one detected absorber and the total number of quasars that can be used to detect absorptions with given criteria. We find that, for both C IV and Mg II absorbers, both the number density of absorbers in given velocity intervals (dn/d\beta) and the f_c show very significant excess at the low-velocity offset from the quasars, relative to the random occurrence that is expected for cosmologically intervening absorbers. These relative excess extensions for Mg II absorptions are not only evidently related to absorption strength but also to quasar luminosity, while they are mainly constrained within 2000 km/s no matter what quasar luminosity and absorption strength are. In addition, we find that the redshift number density (dn/dz) evolution of Mg II absorbers with v_abs <2000 km/s evidently differs from that with v_abs >2000 km/s. Turning to C IV absorptions, the relative excess extensions of both dn/d\beta and f_c are mainly limited within v_abs<4000 km/s, and depend neither on absorption strength nor on quasar luminosity. And also, the absorbers with v_abs<4000 km/s show obviously different redshift number density evolution from those with v_abs>4000 km/s. We suggest velocity cuts of 4000 km/s and 2000 km/s to define quasar C IV and Mg II associated NALs, respectively.