The Dependence of Tidally-Induced Star Formation on Cluster Density
  A survey of H alpha emission in 320 spiral galaxies in 8 nearby clusters shows an enhancement of circumnuclear starburst emission with increasingly rich clusters. These observations provide convincing evidence that spirals have been transformed into S0s in clusters predominantly by tidal forces, a picture fully in accord with the most recent numerical simulations of clusters. For the richest clusters, the enhancement of starburst emission is greater than would be expected on the basis of increasing galaxy surface density alone, which may explain the anomalous result for the type-galaxy surface density relation found for low richness clusters at intermediate redshift. 
C. Moss and M. Whittle