Key points are not available for this paper at this time.
Wide-field optical and near-IR (JHK) imaging is presented for two rich galaxy clusters: Abell 370 at z = 0. 374 and Abell 851 (Cl 0939+47) at z = 0. 407. The new data are combined to produce colors sampling the 0. 55-1. 65 micron range in the rest frame. Galaxy catalogs selected from the near-IR images are 90% complete to a limiting magnitude approximately 1. 5 mag below K^*^. The resulting samples contain ~100 probable member galaxies per cluster in the central ~2 Mpc. Comparison with Hubble Space Telescope (HST) wide-field planetary camera images yields subsamples of ~70 galaxies in each cluster with morphological types. Analysis of the complete samples and the HST subsamples gives the following results: 1. The optical - K colors of the cluster galaxies are bounded by a red envelope of E/S0's which follow a color-magnitude relation with a slope similar to the corresponding color-magnitude relation in present-epoch E/S0's. 2. The z ~ 0. 4 E/S0's are bluer than those in the Bower, Lucey, & Ellis (1992) Coma sample in the optical-K color by 0. 13 mag for Abell 370 and by 0. 18 mag for Abell 851. Our estimate of the systematic uncertainty indicates this color difference is significant at the 2-3 σ level. If real, the bluing of the E/S0 populations at moderate redshift is consistent with that calculated from the Bruzual & Charlot (1993) models of passive elliptical galaxy evolution. 3. In both clusters the intrinsic scatter of the known E/S0's about their optical - K color-magnitude relation is small (~0. 06 mag) and not significantly different from that of Coma E/S0's as given by Bower et al. (1992), indicating that the galaxies within each cluster formed at the same time at an early epoch. These results support the paradigm in which the stellar population in cluster E/S0's is a function only of the galaxy mass and the look-back time. 4. Although the passive-evolution models fit the z ~ 0. 4 early-type galaxies in the optical - K color, they fail to reproduce the observations at intervening wavelengths. When normalized at rest frame 1. 6 micron, the z ~ 0. 4 E/S0's are fainter than the model at rest frame 0. 9 microns, and brighter than the model (and than Coma E/S0's) at rest frame 1. 2 rim. Such color differences are difficult to explain in terms of age or conventional metallicity effects. 5. The disk galaxies in Abell 851 are bluer and represent a greater fraction of the total sample compared to those in Abell 370. X-ray maps and large-field optical images indicate that Abell 370 is "older" than Abell 851, in the sense of showing a more relaxed distribution of galaxies and hot gas. In agreement with the apparent difference in cluster age, our results suggest the disk galaxies of Abell 370 have had more time in the cluster environment, relative to the disks in Abell 851, to redden and/or fade into obscurity.
Stanford et al. (Fri,) studied this question.