Collective properties of quasar narrow associated absorption lines
  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.
