Galaxy Evolution in thez= 0.4274 Cluster MS 1621.5+2640

We discuss the galaxy population of the rich cluster MS1621.5+2640 atz=0.4274, based on spectra and imaging in a field of size 9 arcmin by 23 arcmin(~2 by 5 h**-1 Mpc). The sample comprises 277 galaxies, of which 112 arecluster members, 7 are `near-members', and 47 are field galaxies in theredshift range 0.37<=z<=0.50. The results are analyzed and compared with thez=0.2279 rich cluster Abell 2390. MS1621.5+2640 has a higher blue fraction, ayounger stellar population, and is a less evolved cluster. We do not findstrong evidence of significant excess star formation compared with the field,although there is a small population of outlying near-members that is unusuallyblue and that may be affected by the cluster. There is a substantial populationof red galaxies with significant Hdelta absorption, which are not easilyexplained by any simple form of modeled star formation history. We detect twodistinct populations of cluster galaxies: those where star formation ceasedsome time ago, and those with a gradual decrease over many Gyr. Our datasuggests that the cluster formed by accretion from the field, with truncationof the star formation beginning at very large radii (>2 times the virialradius). The truncation process does not seem to be a sharp one though, in thatlower-luminosity early-type galaxies in the inner core of the cluster are seenwith significant Hdelta absorption, indicating some star formation within thelast 1-2 Gyr. Some combination of stripping of gas from the outer parts of thegalaxy, together with gradual exhaustion of the gas in the inner parts would beconsistent with our data.