Clusters of Galaxies Masquerading as X-Ray Quasars

Abstract

Abstract Inspired by the discovery of the Phoenix cluster by the South Pole Telescope team, we initiated a search for other massive clusters of galaxies missing from the standard X-ray catalogs. We began by identifying 25 cluster candidates not included in the Meta-Catalog of X-ray Clusters of galaxies cluster compilation through cross-identification of the central galaxies of optically identified clusters in the Sloan Digital Sky Survey GMBCG catalog with bright X-ray sources in the ROSAT Bright Source Catalog. Those candidates were mostly unidentified or previously classified as X-ray active galactic nucleus (AGN). We analyzed brief Chandra X-ray Observatory observations of 14 of these X-ray sources and found that eight are X-ray luminous clusters of galaxies, only one showing evidence for a central X-ray point source. The remaining six candidates turned out to be point-source dominated, with faint detections or upper limits on any extended emission. We were not able to rule out the presence of extended X-ray emission from any of the point sources. The levels of extended emission around the six point sources are consistent with expectations based on optical richness, but could also be contaminated by scattered X-ray light from the central point source or extended nonthermal emission from possible radio lobes. We characterize the extended components of each of the well-detected cluster sources, finding that six of the eight X-ray clusters are consistent with being compact cool-core clusters. One of the newly identified low-luminosity X-ray clusters may have had an X-ray-luminous AGN 20 yr prior to the recent Chandra observations, based on the 4σ difference between its Chandra and ROSAT fluxes.