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C. Moss (Vatican Observatory Research Group, Steward Observatory, University of Arizona.), M. Whittle (Department of Astronomy, University of Virginia.)
A survey of H\alpha emission has been made of over 900 galaxies in nine low-redshift Abell clusters, and in the surrounding supercluster field. For galaxies of types Sa and later, there is an enhancement of circumnuclear starburst emission, most likely due to tidal interactions, which correlates both with local galaxy surface density, and with cluster central galaxy density. In the richest cluster surveyed (Coma), the fraction of these galaxies undergoing tidal distortion and/or tidally induced star formation appears comparable to the fraction of spirals showing these effects in rich clusters at z ~0.5. These results suggest that the transformation of spirals to S0s in the cluster populations since z ~0.5 has been effected mainly by tidal forces (whether galaxy--galaxy, galaxy--group or galaxy--cluster interactions) in accord with predictions of numerical simulations of clusters with a non-static potential undergoing collapse and infall. Moreover the survey results indicate that the starburst enhancement is greater (and the implied morphological transformation is faster) in the richer clusters. This may account for the anomalous lack of a morphological type -- galaxy local surface density (T--\Sigma) relation for irregular clusters at intermediate redshift, since it is likely there has been insufficient time for substantial morphological transformation of disk galaxies in these clusters as compared to regular clusters and low-redshift irregular clusters.