Mg II absorption in a sample of 56 steep-spectrum quasars

We present an analysis of the statistical properties of Mg II absorbers found in the spectra of 56 intrinsically faint, steep-spectrum radio quasars. We observe for the first time a significant excess of associated Mg II absorbers over the number expected from cosmologically distributed absorbers. This result is in contrast to previous Mg II surveys in which the QSOs were optically selected. This distinction is similar to the result for associated C IV absorbers, in which intrinsically faint, steep-spectrum quasars show excess associated absorption and intrinsically bright QSOs (both radio-loud and radio-quiet) do not show an excess. From our spectra a statistically complete list of absorption lines is derived, and we find 29 Mg II absorbers, 18 of which have not been previously reported. We also determine several characteristics of the quasar emission lines in our spectra. The Mg II absorber distribution as a function of redshift and equivalent width is calculated both for our sample alone and from our sample combined with spectra from other surveys. For the redshift distribution n(z) = n(1 + z)^γ, we obtain, using the combined sample, the values γ = 1.11 ± 0.46 for W_min = 0.6 Å and γ= 2.47 ± 0.68 for W_min = 1.0 Å. We find that the distribution of strong absorbers is inconsistent with no evolution at a confidence level between 2.2 and 2.9 σ, depending on the deceleration parameter q₀. The deviation from no evolution is similar to what had been previously reported.