The WARPS Survey. II. The log N –log S Relation and the X‐Ray Evolution of Low‐Luminosity Clusters of Galaxies

Abstract

The strong negative evolution observed in previous X-rayÈselected surveys of clusters of galaxies is evidence in favor of hierarchical models of the growth of structure in the universe. A large recent survey has, however, contradicted the low-redshift results, Ðnding no evidence for evolution at z \ 0.3. Here we present the Ðrst results from an X-rayÈselected, Ñux-and surface brightnessÈlimited deep survey for high-redshift clusters and groups of galaxies based on ROSAT PSPC pointed data. The log NÈlog S relation of all clusters in this survey is consistent with that of most previous surveys, but it occupies a Ñux range not previously covered (greater than 6 ] 10~14 ergs cm~2 s~1 total Ñux in the 0.5È2 keV band). At high redshifts (z [ 0.3) the cluster luminosities are in the range 4 ] 1043È2 ] 1044 ergs s~1, the lumi-h 50 ~2 nosities of poor clusters. The number of high-redshift, low-luminosity clusters is consistent with no evolution of the X-ray luminosity function between redshifts of z B 0.4 and z \ 0, and it places a limit of a factor of less than 1.7 (at 90% conÐdence) on the amplitude of any pure negative density evolution of clusters of these luminosities, in contrast with the factor of B3 [corresponding to number density evolution P(1 ] z)~2.5] found in the Einstein Extended Medium-Sensitivity Survey at similar redshifts but higher luminosities. Taken together, these results support hierarchical models in which there is mild negative evolution of the most luminous clusters at high redshift, but little or no evolution of the less luminous but more common optically poor clusters. Models involving preheating of the X-ray gas at an early epoch Ðt the observations, at least for) 0 \ 1.