Enhanced interactions, mergers and starbursts in dense cluster   environments
  Optical and X-ray studies have established the prevalence of significant substructure in clusters of galaxies, indicating that clusters are young systems, and that recent major mergers have occurred in many clusters. Numerical simulations show that sub-cluster merging will result in significant tidal forces on disk galaxies in accreting sub-groups, which is likely to lead to the transformation of spirals to S0s in clusters. Simulations predict simultaneous moderate starbursts in gas-rich disk galaxies in clusters which show on-going or recent merger activity, as well as enhanced galaxy-galaxy interactions and mergers. Observational data from studies of spectral indicators, both of starburst activity in late-type galaxies in nearby clusters, and of poststarburst activity in early-type galaxies in nearby and intermediate redshift clusters, support this scenario. 
C. Moss (Liverpool John Moores University, U.K.)