THE IDENTIFICATION OF QUASARS BEHIND ELLIPTIC GALAXIES AND CLUSTERS OF GALAXIES

The detection of resonance absorption lines against known objects such as individual galaxies and clusters of galaxies is a powerful approac h for studying the gas content of these systems. We describe an effici ent method of identifying background quasars suitable for absorption-l ine studies. In this finding technique, we identify serendipitous X-ra y sources, about one-eighth of which are suitably bright quasars at mo derate redshift. We identify 16 new quasars and galaxies with active g alactic nuclei (AGNs) and confirm five known quasars and AGNs, superpo sed behind elliptical galaxies and clusters of galaxies. We also prese nt three QSO/AGN candidates with uncertain redshift identifications.