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Description
We studied a sample of 415 associated (z_abs_~z_em_; relative velocity with respect to QSO in units of c, {beta}<0.01) MgII 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 ({beta}>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 MgII absorbers have higher ionization (higher ratios of the strengths of CIV and MgII lines), than the intervening absorbers. The ionization decreases with increasing {beta}. 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{AA} absorption 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.
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