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We have studied a sample of 415 associated (z_ab z_em; relative velocity withrespect to QSO <3000km/s) Mg II absorption systems with 1.0<=z_ab<=1.86, in thespectra of SDSS DR3 QSOs, to determine the dust content and ionization state inthe absorbers. We studied the dependence of these properties on the propertiesof the QSOs and also, compared the properties with those of a similarlyselected sample of 809 intervening systems (apparent relative velocity withrespect to the QSO of >3000km/s), so as to understand their origin. From theanalysis of the composite spectra, as well as from the comparison of measuredequivalent widths in individual spectra, we conclude that the associated Mg IIabsorbers have higher apparent ionization, measured by the strength of the C IVabsorption lines compared to the Mg II absorption lines, than the interveningabsorbers. The ionization so measured appears to be related to apparentejection velocity, being lower as the apparent ejection velocity is more andmore positive. There is clear evidence, from the composite spectra, for SMClike dust attenuation in these systems; the 2175AA absorption feature is notpresent. The extinction is almost twice that observed in the similarly selectedsample of intervening systems. We reconfirm that QSOs with non-zero FIRST radioflux are intrinsically redder than the QSOs with no detection in the FIRSTsurvey. The incidence of associated Mg II systems in QSOs with non-zero FIRSTradio flux is 1.7 times that in the QSOs with no detection in the FIRST survey.The associated absorbers in radio-detected QSOs which comprise about 12% of oursample, cause 3 times more reddening than the associated absorbers inradio-undetected QSOs. This excess reddening possibly suggests an intrinsicnature for the associated absorbers in radio-detected QSOs.

Average Properties of a Large Sample of z_abs ~ z_em associated Mg II Absorption Line Systems