2001ApJ...548L.143M

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 f_B is real and occurs at approximately the same rate for clusters of all richness classes.