Evolution of Cluster X-ray Luminosities and Radii: Results from the 160 Square Degree Rosat Survey

We searched for cluster X-ray luminosity and radius evolution using our sample of 201 galaxy clusters detected in the 160 deg 2 survey with the ROSAT PSPC (Vikhlinin et al. 1998). With such a large area survey, it is possible, for the first time with ROSAT, to test the evolution of luminous clusters, L x > 3 × 10 44 ergs s −1 in the 0.5–2 keV band. We detect a factor of 3–4 deficit of such luminous clusters at z > 0.3 compared to the present. The evolution is much weaker or absent at modestly lower luminosities, 1–3 × 10 44 ergs s −1. At still lower luminosities, we find no evolution from the analysis of the log N − log S relation. The results in the two upper L x bins are in agreement with the Einstein EMSS evolution result (Gioia et al. 1990a, Henry et al. 1992) while being obtained using a completely independent cluster sample. The low-L x results are in agreement with other ROSAT surveys (e.g. Rosati et al. 1998, Jones et al. 1998). We also compare the distribution of core radii of nearby and distant (z > 0.4) luminous (with equivalent temperatures 4–7 keV) clusters, and detect no evolution. The ratio of average core radius for z ∼ 0.5 and z < 0.1 clusters is 0.9 ± 0.1, and the core radius distributions are remarkably similar. A decrease of cluster sizes incompatible with our data is predicted by self-similar evolution models for high-Ω universe.