The discovery of two distant, massive clusters of galaxies in the ROSAT All-Sky Survey

We discuss the radio, optical, and X-ray properties of two newly discovered, very X-ray luminous, distant clusters of galaxies. Both systems were noted as cluster candidates in a cross-correlation of data from the WENSS radio survey and the ROSAT All-Sky Survey. Follow-up observations performed by us and the Massive Cluster Survey (MACS) team confirmed both sources as distant galaxy clusters. The first cluster, MACS J0717.5+3745 at a redshift of z = 0.5548, contains a very extended, steep-spectrum radio source offset from the cluster core, making it the most distant radio relic known. The second cluster, MACS J1621.3+3810 at z = 0.465, is a strong cooling flow with a relatively weak central radio source. We present results from ROSAT High-Resolution Imager (HRI) observations of both clusters as well as from optical imaging and VLA radio interferometry observations. Our discoveries demonstrate that distant clusters can be efficiently identified in a relatively shallow X-ray survey, that radio/X-ray selection is efficient, and that both cooling flow and non-cooling flow clusters are selected.