ROSAT Detections at High Redshift

Abstract

New ROSAT observations of distant clusters of galaxies are presented. Two programmes are underway. Deep pointed exposures are being gathered for a complete sub-sample of the Couch et al. southern cluster catalogue. The mean redshift of this sub-sample is z = 0.42 and preliminary results indicate that the bulk of these concentrated clusters, identified on prime focus AAT plates, have X-ray luminosities less than 5 × 10⁴³ h₅₀ ^-2 ergs^-1 (h ₅₀ ≡ H ₀ / 50 kms^-1 Mpc^-1). A second programme is concerned with deeper X-ray exposures of the most distant z > 0.6 optically-rich clusters from the catalogue of Gunn, Hoessel and Oke. So far, 2 out of 5 clusters are detected with X-ray luminosities of ≃ 10⁴⁴ h ₅₀ ^-2 ergs^-1, much lower than those of present-day rich clusters. By considering the likely volume sampled we find evidence in both samples for a decline in the comoving number density of clusters with redshift. Our results are inconsistent with standard hierarchical clustering models in which the gas evolves in a self-similar fashion, indicating that radiative and hydro dynamic processes may be required to account for the low observed luminosities.