The Butcher-Oemler Effect in 295 Clusters: Strong Redshift Evolution and Cluster Richness Dependence
20 February 2001
We examine the Butcher-Oemler effect and its cluster richness dependence in the largest sample studied to date: 295 Abell clusters. We find a strong correlation between cluster richness and the fraction of blue galaxies, f sub Bm at every redshift. The slope of the f sub B (z) relation is the same for all richnesses, but at a given redshift, f sub B is systematically higher for poor clusters. This is the chief cause of scatter in the f sub B vs., z diagram: the spread caused by the richness dependence is comparable to the trend in f sub B over a typical redshift baseline, so that conclusions drawn from smaller samples have varied widely. The two parameters, z, and a consistently defined projected galaxy number density, N, together account for all of the observed variation in f sub B within the measurement errors. The redshift evolution of f sub B is real, and occurs at the same rate for clusters of all richness classes.