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 c luster evolution and to explore the potential implications for large-scale structure models. The survey is 99.6% optically identified. Spectroscopic r edshifts have been measured for all the extragalactic identifications. In t his Letter, first results on cluster evolution are presented based on a com parison between the number of the observed clusters in the north ecliptic p ole 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 univ erse that is significant at greater than 4.7 sigma. The evolution appears t o commence at L0.5-2.0 keV > 1.8 x 10(44) ergs s(-1) in our data. The negat ive evolution goes in the same direction as the original Extended Medium-Se nsitivity Survey result, the results from the 160 deg(2) survey, and the re cent results from the ROSAT Deep Cluster Survey. At lower redshifts, there is no evidence for evolution, a result in agreement with these and other cl uster surveys.