The Clusters Hiding in Plain Sight (CHiPS) Survey

Even though performing optical surveys to find galaxy clusters has become relatively cheap with the introduction of a new generation of large wide field optical telescopes, X-ray surveys remain a cornerstone of galaxy cluster science. One of the main reasons is that X-ray-selected cluster samples are less affected by projection effects, compared to the optical catalogs. There are several well established X-ray cluster catalogs based on the ROSAT All-Sky Survey (RASS), including REFLEX, NORAS, BCS, SGP, and MACS. However with limited resolution and sensitivity, all of these surveys had challenges at high redshifts to distinguish sharply peaked cool core clusters from isolated AGN, leading some cool core clusters to be misidentified as bright point sources in the RASS catalog.

In this talk, I will present the Cluster Hiding in Plain Sight (CHiPS) survey, which is our attempt to discover galaxy clusters that have been misclassified as point sources in the RASS dataset. We achieve this by conducting an extensive optical follow-up of the RASS point source catalog to look for galaxy overdensities at the location of the point sources. The CHiPS survey has resulted in two newly discovered clusters–CHIPS 1356-3421 and CHIPS 1911+4455–which are massive enough to be detected in previous galaxy cluster catalogs. The brightest cluster galaxy (BCG) in CHIPS 1911+4455 hosts a massive starburst (SFR~180 Msun/yr). The X-ray emission from the cluster is sharply peaked onto the BCG; however, somewhat unusually, the large-scale X-ray morphology is highly asymmetric, pointing to a dynamically active cluster. CHiPS serves as a pathfinder for similar work on larger scales that will be necessary for a full understanding of the cluster population detected with eROSITA, the successor to ROSAT.