A Search for X–ray Bright Distant Clusters of Galaxies

We present the results of a search for X–ray luminous distant clusters of galaxies. We found extended X–ray emission characteristic of a cluster towards two of our candidate clusters of galaxies. They both have a luminosity in the ROSAT bandpass of ≃ 1044 erg s−1 and a redshift of> 0.5; thus making them two of the most distant X–ray clusters ever observed. Furthermore, we show that both clusters are optically rich and have a known radio source associated with them. We compare our result with other recent searches for distant X–ray luminous clusters and present a lower limit of 1.2 × 10−7 Mpc−3 for the number density of such high redshift clusters. This limit is consistent with the expected abundance of such clusters in a standard (b=2) Cold Dark Matter Universe. Finally, our clusters provide important high redshift targets for further study into the origin and evolution of massive clusters of galaxies