Absorption lines in the spectra of low-redshift quasi-stellar objects

In earlier investigations, the appearance of Mg II absorption was reported in the spectra of seven low-redshift (z/sub em/< or approx. =1) QSOs. On the basis of new higher-dispersion observations, the existence of of Mg II absorption is verified in the spectra of only two of these QSOs, PKS 1229--02 (z/sub abs/very-much-less-thanz/sub em/) and 4C 28.25 (z/sub abs/approx. =z/sub em/). From these data and from those of a continuing survey at Steward Observatory, it is found that Mg II absorption occurs only rarely near the emission redshift; this contrasts sharply with the frequency with which C IV absorption occurs near the emission redshift in the spectra of higher-redshift QSOs. The implications of this difference are discussed. The absence of Mg II absorption at the emission redshift also provides a constraint for models of QSO emission-line regions that is quite strong for photoionization modesl which are optically thick to Lyman continuum radiation.