EVOLUTION OF INFRARED-SELECTED GALAXIES IN Z-SIMILAR-TO-0.4 CLUSTERS

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 mu m range in the rest frame. Galaxy catalogs selected from the near-IR images are 90% complete to a limiting magnitude approxima tely 1.5 mag below K. The resulting samples contain similar to 100 pr obable member galaxies per cluster in the central similar to 2 Mpc. Co mparison with Hubble Space Telescope (HST) wide-field planetary camera images yields subsamples of similar to 70 galaxies in each cluster wi th morphological types. Analysis of the complete samples and the HST s ubsamples gives the following results: 1. The optical-K colors of the cluster galaxies are bounded by a red envelope of E/SO's which follow a color-magnitude relation with a slope similar to the corresponding c olor-magnitude relation in present-epoch E/SO's. 2. The z similar to 0 .4 E/SO's are bluer than those in the Bower, Lucey, and Ellis (1992) C oma sample in the optical-K color by 0.13 mag for Abell 370 and by 0.1 8 mag for Abell 851. Our estimate of the systematic uncertainty indica tes this color difference is significant at the 2-3 sigma level. If re al, the bluing of the E/SO populations at moderate redshift is consist ent with that calculated from the Bruzual and Chariot (1993) models of passive elliptical galaxy evolution. 3. In both clusters the intrinsi c scatter of the known E/SO's about their optical - K color-magnitude relation is small (similar to 0.06 mag) and not significantly differen t from that of Coma E/SO's as given by Bower et al. (1992), indicating that the galaxies within each cluster formed at the same time at an e arly epoch. These results support the paradigm in which the stellar po pulation in cluster E/SO's is a function only of the galaxy mass and t he look-back time. 4. Although the passive-evolution models fit the z similar to 0.4 early-type galaxies in the optical-K color, they fail t o reproduce the observations at intervening wavelengths. When normaliz ed at rest frame similar to 1.6 mu m, the z similar to 0.4 E/SO's are fainter than the model at rest frame similar to 0.9 mu m, and brighter than the model (and than Coma E/SO's) at rest frame similar to 1.2 mu m. Such color differences are difficult to explain in terms of age or conventional metallicity effects. 5. The disk galaxies in Abell 851 a re bluer and represent a greater fraction of the total sample compared to those in Abell 370. X-ray maps and large-field optical images indi cate that Abell 370 is ''older'' than Abell 851, in the sense of showi ng a more relaxed distribution of galaxies and hot gas. In agreement w ith the apparent difference in cluster age, our results suggest the di sk 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 obscu rity.