Dynamical effects of galaxy encounters

The tidal interactions during close encounters may cause changes in the kinematical and structural properties of galaxies. Fast interpenetrating encounters are studied by a softened impulse approximation. On this basis, the cumulative tidal effects on a typical cluster or group galaxy are calculated and the results compared with detailed surface photometry of edge-on spiral and SO galaxies. This comparison suggests that tidal effects in most present-day environments are weak and that spiral and lenticular galaxies have formed under different conditions. To study slow encounters and the merging of galaxies numerical methods are used. An N-body disc galaxy model is constructed and its properties explored. (author)