ON THE NATURE OF MG-II ABSORPTION-LINE SYSTEMS IN QUASARS

The results of a large R-band imaging survey of 71 bright (m(V) < 18) quasars are presented. The quasars were chosen from published samples which have intermediate resolution optical spectroscopy available, so the presence of low redshift Mg II absorption lines can be determined. We have searched our data for galaxies close to the line-of-sight to the quasars, which we might be able to identify with the absorption sy stems. We find a high coincidence between galaxies very near the line- of-sight and quasars showing absorption systems in their spectra, a re sult consistent with other studies. These galaxies have a mean luminos ity of 0.5L (assuming they lie at the absorption redshift). The distr ibution of impact parameters between the galaxies and the quasars exte nds with a flat distribution to large radii ( > 30h-1 kpc). This sugge sts that the absorption systems may not be gravitationally bound to th e observed galaxies, but may be part of larger extended systems. We al so find a significant number of galaxies near the line-of-sight to the quasar where no absorption is seen in the quasar spectrum. The select ion of our quasars is unbiased with respect to galaxies near the line- of-sight, so we can compare the observed number of absorption systems to that predicted by a simple model with a constant covering factor in Mg II absorbing gas within a maximum radius of each detected galaxy. The model is consistent with a covering factor of unity, but allowing for incompleteness in the detection of galaxies, the covering factor i s less than unity. The redshifts of the galaxies are required to confi rm this result.