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Session 6 - Evolution, Survey and Clusters of Galaxies.
Display session, Monday, June 10
Tripp Commons,
Twenty of the 79 z>0.2 distant, optically selected, cluster candidates in the Palomar Deep Cluster Survey (Postman et al. 1996) were serendipitously observed by the ROSAT satellite. We have reduced the three PSPC pointings that overlapped Postman et al. (1996) fields using the Snowden et al. (1994) Extended X-ray Analysis Software and a wavelet analysis source detection algorithm. We derive count rates for the cluster candidates inside a metric aperture defined by the estimate redshift quoted in Postman et al. (1996). We find all 20 clusters observed by the PSPC to be X-ray faint; only 4 were detected at >3\sigma. The luminosities derived from the measured/upper-limit count rates are significantly lower than those predicted from the richness vs luminosity distribution derived for X-ray selected clusters (Edge amp; Stewart 1991). This discrepancy cannot be wholly attributed to projection effects; according to Postman et al. (1996) less than 30% of their objects are ``phantom clusters''. We will present evidence to support our conclusion that optically selected clusters are younger and less evolved than their X-ray selected counterparts.
