Interaction of Galaxies with the ICM

Although speculations of an interaction between galaxies and the ICM date back more than thirty years, the impact and importance of a possible interaction have long remained elusive. In recent years the situation has completely changed. A wealth of data and detailed hydrodynamical simulations have appeared that show the effects of interactions. Single dish observations show that cluster galaxies are deficient in their neutral hydrogen content out to two Abell radii. The deficient galaxies tend to be on radial orbits. Detailed imaging of the neutral hydrogen distribution in individual galaxies in two nearby clusters show a remarkable trend of H I extent with location in the cluster. These trends can be reproduced in simulations of ram pressure stripping by the ICM using SPH and full 3D hydro-codes. Detailed imaging of individual galaxies have found a number of galaxies with undisturbed stellar disks, truncated gas disks that are much smaller than the stellar disks, asymmetric extraplanar gas in the center and enhanced central star formation. These phenomena have all been predicted by hydrodynamical simulations. For the first time detailed observations of gas morphology and kinematics are used to constrain simulations. Simple models of ram pressure stripping are consistent with the data for some galaxies, while for other galaxies more than one mechanism must be at work. Optical imaging and spectroscopic surveys show that small H I disks go together with truncated star forming disks, that hydrogen deficiency correlates with suppressed star formation rates and that the spatial extent of H I deficiency in clusters is matched by or even surpassed by the extent of reduced star formation rates. Recent volume limited imaging surveys of clusters in the local universe show that most gas rich galaxies are located in smaller groups and subclumps, that yet have to fall into the clusters. These groups form an ideal environment for interactions and mergers to occur and we see much evidence for interactions between gas rich galaxies.