GALAXY EVOLUTION IN ABELL-2390

The galaxy population in the intermediate-redshift (z=0.228) rich clus ter Abell 2390 is investigated. We present velocities, colors, and mor phological information for an exceptionally large sample of 323 galaxi es (216 cluster members) in a 46' x 7' (6 h(-1) Mpc x 1 h(-1) Mpc) str ip centered on the cD galaxy. This sample of confirmed cluster members is second only to that for the Coma cluster in terms of sample size a nd spatial coverage in the cluster rest frame and it is the first to t race the transition between a rich cluster and the field at intermedia te redshift. The galaxy population in the cluster changes gradually fr om a very evolved, early-type population in the inner 0.4 h(-1) Mpc of the cluster to a progressively later type population in the extensive outer envelope of the cluster from 1 to 3 h(-1) Mpc in radius. Radial gradients in galaxy g-r color, 4000 Angstrom break, H delta and [O II ] line strengths, and morphology are seen in the cluster and are inves tigated by comparing the data to models computed with the GISSEL spect ral synthesis package. The results suggest that the cluster has been b uilt up gradually by the infall of field galaxies over similar to 8 Gy r and that star formation has been truncated in infalling galaxies dur ing the accretion process. The morphological composition of the cluste r is shown to be consistent with such a scenario. If true for other cl usters, infall-truncated star formation as seen in Abell 2390 may expl ain both the Butcher-Oemler effect and the large fraction of S0 galaxi es in clusters. Only less than or similar to 5% of the galaxies observ ed in Abell 2390 exhibit evidence for star formation at levels stronge r than those seen in typical late-type systems. This suggests that sta rbursts do not play a major role in driving cluster galaxy evolution a t the redshift of Abell 2390, although infall-induced starbursts leadi ng to truncated star formation may have played a role in the earlier h istory of the cluster. Evidence is found for at least one sub-componen t on the west sidecluster, which is likely to be infalling at the epoc h of observation.