Ve. Margoniner et al., The Butcher-Oemler effect in 295 clusters: Strong redshift evolution and cluster richness dependence, ASTROPHYS J, 548(2), 2001, pp. L143-L146
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 c
orrelation 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 ri
chnesses, but at a given redshift, f(B) is systematically higher for poor c
lusters. This is the chief cause of scatter in the f(B) versus z diagram; t
he 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 defin
ed projected galaxy number density N, together account for all of the obser
ved 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 r
ichness classes.