Intermediate-redshift galaxy halos: Results from QSO absorption lines

For a sample of Mg II-selected QSO absorption-line systems for which the absorbing galaxies have been successfully identified, we examine the rest-frame equivalent widths of the Mg II λ2796 absorption lines as a function of the known impact parameters between the background QSOs and the absorbing galaxies. There appears to exist a relationship between the equivalent widths and the impact parameters, in the sense that larger equivalent widths occur at smaller impact parameters. No trend of the doublet ratio is found with impact parameter, and neither the equivalent widths nor the doublet ratios are correlated with the absolute luminosities or redshifts of the absorbing galaxies. These results apparently indicate that the main factor that determines the equivalent width of a particular absorption system is the impact parameter between the background QSO and the absorbing galaxy. We interpret the relationship between equivalent width and impact parameter as indicating the spatial density of absorbing clouds within the extended halo regions of the absorbing galaxies, and we determine a radial profile of the spatial density of absorbing clouds roughly proportional to r^-(α+1), with α in the range α ≈ 1-2.