The Evolution of Population Gradients in Galaxy Clusters: The Butcher‐Oemler Effect and Cluster Infall

We present photometric and spectroscopic measurements of the galaxypopulations in clusters from the CNOC1 sample of rich, X-ray luminous clustersat $0.18 < z < 0.55$. A classical measure of the galaxy blue fraction forspectroscopically confirmed cluster members shows a significant Butcher-Oemlereffect for the sample, but only when radii larger than 0.5$r_{200}$ areconsidered. We perform a principal component analysis of galaxy spectra todivide the total cluster light into contributions from stellar populations ofdifferent ages. Composite radial distributions of different stellar populationsshow strong gradients as a function of cluster-centric radius. The compositepopulation is dominated by evolved populations in the core, and graduallychanges at radii greater than the virial radius to one which is similar tocoeval field galaxies. We do not see evidence at any radius within the clustersfor an excess of star formation over that seen in the coeval field. Within thisredshift range, significant evolution in the fractional population gradient isseen. Both low and high redshift clusters have similar populations in thecluster cores, but higher redshift clusters have steeper gradients and morestar forming galaxies at radii outside of the core region~-- in effect, arestatement of the Butcher-Oemler effect. Luminosity density profiles areconsistent with a scenario where this phenomenon is due to a decline over timein the infall rate of field galaxies into clusters. Depending on how longgalaxies reside in clusters before their star formation rates are diminished,this suggests a decrease in the infall into clusters of a factor of $\sim 3$between $z > 0.8$ and $z \sim 0.5$. We also discuss alternative scenarios forthe evolution of cluster populations.Comment: ApJ, in pres