The redshift distribution of intervening weak mg ii quasar absorbers and a curious dependence on quasar luminosity

Abstract

We have identified 469 Mg II λλ2796, 2803 doublet systems having Wr ≥ 0.02 Å in 252 Keck/High Resolution Echelle Spectrometer and UVES/Very Large Telescope quasar spectra over the redshift range 0.1 < z < 2.6. Using the largest sample yet of 188 weak Mg II systems (0.02 Å ≤Wr < 0.3 Å), we calculate their absorber redshift path density, dN/dz. We find clear evidence of evolution, with dN/dz peaking at z ∼ 1.2, and that the product of the absorber number density and cross section decreases linearly with increasing redshift; weak Mg II absorbers seem to vanish above z ≃ 2.7. If the absorbers are ionized by the UV background, we estimate number densities of 106-109 Mpc-3 for spherical geometries and 102-105 Mpc-3 for more sheetlike geometries. We also find that dN/dz toward intrinsically faint versus bright quasars differs significantly for weak and strong (Wr ≥ 1.0 Å) absorbers. For weak absorption, dN/dz toward bright quasars is ∼25% higher than toward faint quasars (10σ at low redshift, 0.4 ≤ z ≤ 1.4, and 4σ at high redshift, 1.4