2001/05/14 by B. R. McNamara, A. Vikhlinin, A. Hornstrup +4 · 2 citations
Physics and Astronomy · #Astronomy and Astrophysical Research #Galaxies: Formation, Evolution, Phenomena #Scientific Research and Discoveries #astro-ph
paper · pdf · doi:10.1086/322306
The abstract has been abridged. Accepted for publication in the Astrophysical Journal
arxiv created 2001/05/14 · openalex publication_date 2001/09/10 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/07/31
We have measured the surface density of galaxies toward 14 X-ray-selected cluster candidates at redshifts z ≳ 0.46, and we show that they are associated with rich galaxy concentrations. These clusters, having X-ray luminosities of L X (0.5-2) ~ (0.5-2.6) × 10 44 ergs s -1 , are among the most distant and luminous in our 160 deg 2 ROSAT Position Sensitive Proportional Counter cluster survey. We find that the clusters range between Abell richness classes 0 and 2 and have a most probable richness class of 1. We compare the richness distribution of our distant clusters to those for three samples of nearby clusters with similar X-ray luminosities. We find that the nearby and distant samples have similar richness distributions, which shows that clusters have apparently not evolved substantially in richness since redshift z = 0.5. There is, however, a marginal tendency for the distant clusters to be slightly poorer than nearby clusters, although deeper multicolor data for a large sample would be required to confirm this trend. We compare the distribution of distant X-ray clusters in the L X -richness plane to the distribution of optically selected clusters from the Palomar Distant Cluster Survey. The optically selected clusters appear overly rich for their X-ray luminosities when compared to X-ray-selected clusters. Apparently, X-ray and optical surveys do not necessarily sample identical mass concentrations at large redshifts. This may indicate the existence of a population of optically rich clusters with anomalously low X-ray emission. More likely, however, it reflects the tendency for optical surveys to select unvirialized mass concentrations, as might be expected when peering along large-scale filaments.