High-Resolution Spectra of 24 Low-Redshift QSOs: The Properties of MG II Absorption Systems

High-resolution spectra of 245 low-redshift QSOs are presented, and Mg II emission-line equivalent widths and velocity widths are measured and briefly discussed. Improved statistical methods are presented for the extraction of significant absorption features from spectra. These techniques provide a simple and accurate estimate of the minimum equivalent widths which could be detected in any region of a spectrum. A search for Galactic Ca II in 13 lines of sight reveals five detections. Equivalent widths are in the range 0.2-0.3 A. Only three redshifted absorption systems were found, all dominated by Mg II 2796 A, 2803 A doublets. Mg(+)/Mg(0) and Mg(+)/Fe(+) values cannot be accounted for by collisional ionization alone. Factors determining the frequency of occurrence of Mg II systems are investigated using a composite sample of 14 systems from the spectra of 90 QSOs. Ten out of the 14 systems occur in five pairs of two systems each per QSO. The probability of these pairs being chance occurrences is only 0.0007.