Cluster Evolution in the ROSAT North Ecliptic Pole Survey

The deepest region of the ROSAT All−Sky Survey, at the North Ecliptic Pole, has been studied to produce a complete and unbiased X-ray selected sample of clusters of galaxies. This sample is used to investigate the nature of cluster evolution and explore potential implications for large-scale structure models. The survey is 99.6% optically identified. Spectroscopic redshifts have been measured for all the extragalactic identifications. In this Letter, first results on cluster evolution are presented based on a comparison between the number of the observed clusters in the North Ecliptic Pole survey and the number of expected clusters assuming no-evolution models. At z > 0.3 there is a deficit of clusters with respect to the local universe which is significant at > 4.7σ. The evolution appears to commence at L0.5−2.0 > 1.8× 10 44 erg s in our data. The negative evolution goes in the same direction as the original EMSS result, the results from the 160 deg survey by Vikhlinin et al. (1998) and the recent results from the RDCS (Rosati et al. 2000). At lower redshifts there is no evidence for evolution, a result in agreement with these and other cluster surveys. Subject headings: galaxies: clusters: general surveys; X-rays: general galaxies