The Evolution of X-ray Selected Clusters of Galaxies

We perform an observational study of cluster evolution by comparing two x-ray selected cluster catalogs. The high redshift catalog consists of an x-ray flux limited sample of clusters from the Extended Medium Sensitivity Survey, with redshifts of 0.3 < z < 0.4. The low redshift catalog consists of an x-ray flux limited sample of clusters from the HEAO-1 A2 survey, with redshifts of 0.03 < z < 0.1. As the selection of clusters by x-ray emission selects on the presence of a deep potential well filled with plasma, the catalogs are free from the inherent superposition problems of optical overdensity selected catalogs, and from biases associated with selecting clusters in the optical for later study in the optical. We expect our catalogs to provide an unbiased view of the evolution of clusters since z=1/3. We have performed a multicolor imaging survey on both the high and low redshift cluster catalogs. The clusters turn out to be very rich; x-ray selection is a very efficient means of finding clusters. We examine the galaxy density in a radius of 0.5 h₇₅ Mpc from the brightest cluster galaxy, and show that it has not changed since z=1/3. The colors of the brightest cluster galaxies do show a variation, with at least half the population scattering blueward from the colors of a passively evolving model. Finally, we show that the Butcher-Oemler effect is not a universal phenomena; roughly 30% of clusters at z=1/3 do not exhibit the classic photometric blueing of the galaxy population. Those clusters with the most relaxed morphology tend to be reddest while those with the most confused morphology tend to be bluest.