Group-cluster merging and the formation of starburst galaxies

A significant fraction of clusters of galaxies are observed to have substru cture, which implies that merging between clusters and subclusters is a rat her common physical process of cluster formation. It still remains unclear how cluster merging affects the evolution of cluster member galaxies. We re port the results of numerical simulations, which show the dynamical evoluti on of a gas-rich late-type spiral in a merger between a small group of gala xies and a cluster. The simulations demonstrate that the time-dependent tid al gravitational field of the merging excites the nonaxisymmetric structure of the galaxy, subsequently drives efficient transfer of gas to the centra l region, and finally triggers a secondary starburst. This result provides not only a new mechanism for starbursts but also a close physical relations hip between the emergence of starburst galaxies and the formation of substr ucture in clusters. We accordingly interpret post-starburst galaxies locate d near substructure of the Coma Cluster as one observational example indica ting the global tidal effects of group-cluster merging. Our numerical resul ts furthermore suggest a causal link. between the observed excess of blue g alaxies in distant clusters and the cluster virialization process through h ierarchical merging of subclusters.