Average properties of a large sample of z_abs ∼ z _em associated Mg II absorption line systems

We studied a sample of 415 associated (z_abs ∼ z _em; relative velocity with respect to QSO in units of c, β< 0.01) Mg il absorption systems with 1.0 ≤ z_abs ≤ 1.86, in the spectra of SDSS DR3 QSOs, to determine the dust content and ionization state in the absorbers. We also compared these properties to those of a similarly selected sample of 809 intervening systems (β > 0.01), so as to understand their origin. Normalized, composite spectra were derived for absorption line measurements, for the full sample and for several subsamples, chosen on the basis of the line strengths and other absorber and QSO properties. From these, and from the equivalent widths in individual spectra, we conclude that the associated Mg II absorbers have higher ionization (higher ratios of the strengths of C IV and Mg II lines), than the intervening absorbers. The ionization decreases with increasing β. Average extinction curves were obtained for the subsamples by comparing their geometric mean QSO spectra with those of matching (in z_em and i magnitude) samples of QSOs without absorption lines. There is clear evidence for SMC-like dust attenuation in these systems; the 2175 Åabsorption feature is absent. The extinction is almost twice that observed in intervening systems. We reconfirm that QSOs with nonzero FIRST radio flux are intrinsically redder than the QSOs with no detection in the FIRST survey. The incidence of associated Mg n systems in radio-detected QSOs is 1.7 times that in radio-undetected QSOs. The associated absorbers in radio-detected QSOs cause 3 times more reddening than those in radio-undetected QSOs. This excess reddening is correlated with the strength of Mg II absorption, possibly suggesting an intrinsic nature for the associated absorbers in radio-detected QSOs.