THE DETECTION OF X-RAY-EMISSION FROM THE HIGHEST REDSHIFT GALAXY CLUSTERS

We present and analyze deep ROSAT observations of two fields containing the most distant (z > 0.7) optically selected clusters of galaxies currently known. We reliably detect X-ray emission from two clusters (including one at z congruent-to 0.9) out of five with available redshifts, but we do not detect any emission from a further five candidates without spectroscopic data. Although our distant clusters are expected to be among the richest found optically, their X-ray luminosities (L(x) congruent-to 10(44) h50(-2) ergs s-1) are much lower than those of present-day rich clusters. We argue that the clusters we have detected are the only X-ray-luminous examples in the fields surveyed. By considering the likely volume sampled we find evidence for a decline in the comoving number density of clusters to z congruent-to 1. On the basis of our current small sample, our results are inconsistent with standard hierarchical clustering models in which the gas evolves in a self-similar fashion, indicating that radiative and hydrodynamic processes may be required to account for the low observed luminosities. For example, a cold dark matter model in which the entropy of the intracluster gas is assumed constant predicts a low abundance of luminous clusters at z congruent-to 0.8 as inferred from our observations.