The Butcher-Oemler effect in 295 clusters: Strong redshift evolution and cluster richness dependence

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(B), at every redshift. The slope of the f(B)(z) relation is similar for all richnesses, but at a given redshift, f(B) is systematically higher for poor clusters. This is the chief cause of scatter in the f(B) versus z diagram; the spread caused by the richness dependence is comparable to the trend in f(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(B) within the measurement errors. The redshift evolution of is real and occurs at approximately the same rate for clusters of all richness classes.