Galaxy evolution in the z=0.4274 cluster MS 1621.5+2640

We discuss the galaxy population of the rich duster MS 1621.5 + 2640 at z = 0.4274, based on spectra and imaging in a field of size 9' x 23' (similar to 2 x 5 h(-1) Mpc). The sample comprises 277 galaxies, of which 112 are cl uster members, 7 are "near-members," and 47 are field galaxies in the redsh ift range 0.37 less than or equal to z less than or equal to 0.50. The resu lts are analyzed and compared with the z = 0.2279 rich cluster Abell 2390. MS 1621.5+2640 has a higher blue fraction and a younger stellar population, and it is a less evolved cluster. We do not find strong evidence of signif icant excess star formation compared with the field, although there is a sm all population of outlying near-members that is unusually blue and that may be affected by the cluster. There is a substantial population of red galax ies with significant H delta absorption, which are not easily explained by any simple form of modeled star formation history. We detect two distinct p opulations of cluster galaxies: those where star formation ceased some time ago, and those with a gradual decrease over many Gyr. Our data suggests th at the cluster formed by accretion from the field, with truncation of the s tar formation beginning at very large radii (>2 times the virial radius). T he truncation process does not seem to be a sharp one though, in that lower luminosity early-type galaxies in the inner core of the cluster are seen w ith significant H delta absorption, indicating some star formation within t he last 1-2 Gyr. Some combination of stripping of gas from the outer parts of the galaxy together with a gradual exhaustion of the gas in the inner pa rts would be consistent with our data.