1 99 9 Population Gradients in Galaxy Clusters at 0

We present a principal component analysis of galaxy spectra from the CNOC sample of rich X-ray luminous clusters at 0.18 < z < 0.55. Composite radial distributions of different stellar populations show strong gradients as a function of cluster-centric redshift. The composite population is dominated by evolved populations in the core, and gradually changes to one which is similar to coeval field galaxies at radii greater than the virial radius. We do not see evidence in the clusters for an excess of star formation over that seen in the coeval field. Within this redshift range, significant evolution in the gradient shape is seen, with higher redshift clusters showing steeper gradients. This results in larger numbers of younger galaxies seen towards the inner regions of the clusters– in effect, a restatement of the Butcher-Oemler effect. Luminosity density profiles are consistent with a scenario where this phenomenon is due to a decline over time in the infall rate of field galaxies into clusters. Depending on how long galaxies reside in clusters before their star formation rates are diminished, this suggests an epoch for maximal infall into clusters at z > 0.7. We also discuss alternative scenarios for the evolution of cluster populations.