[RS2005] ACO 2218 573 , the SIMBAD biblio

We combine new narrowband photometry with archival Wide Field Planetary Camera 2 (WFPC2) data for A2218 (z=0.18) and A2125 (z=0.25), two clusters with intermediate redshifts but very different cluster properties, in order to examine the evolution of galaxy populations. A2218 is a dense, elliptical-rich cluster (Bautz-Morgan type II) similar to the Coma Cluster in its evolutionary appearance, whereas A2125 is a less dense, more open cluster (Bautz-Morgan type II-III), although similar in richness to A2218. The color-magnitude relation indicates that A2125 has a more developed blue population than A2218 (the Butcher-Oemler effect), although both clusters have significant numbers of blue galaxies (ranging in star formation rate from normal, star-forming disks to starburst systems) as compared to a present-day cluster. The colors of the red populations are identical in A2125 and A2218 and are well fitted by passive evolution models. We are able, for the first time, to combine archived WFPC2 images with our narrowband photometry for a color, morphological, and structural analysis of the blue Butcher-Oemler population. We find the blue population to be composed of two subpopulations, a bright, spiral population and a fainter, dwarf starburst population. A2125 is richer in bright starburst systems, a phenomenon apparently induced by the cluster's younger dynamical state. In addition, a majority of the S0 population in A2125 and A2218 is composed of bulge+disk systems, whereas nearby clusters such as Coma are composed primarily of lenticulars (pure disk S0s). The structural parameters of the S0 bulges in A2125 and A2218 are identical to the structure parameters of cluster ellipticals, suggesting that the disks of some S0s in intermediate-redshift clusters are stripped away, with the leftover bulges evolving into present-day ellipticals.