The identification of quasars behind elliptical 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 approach for studying the gas content of these systems. We describe an efficient method of identifying background quasars suitable for absorption-line studies. In this finding technique, we identify serendipitous X-ray sources, about one-eighth of which are suitably bright quasars at moderate redshift. We identify 16 new quasars and galaxies with active galactic nuclei (AGNs) and confirm five known quasars and AGNs, superposed behind elliptical galaxies and clusters of galaxies. We also present three QSO/AGN candidates with uncertain redshift identifications.