TIDAL PERTURBATIONS AND GALAXY SPIRAL ARMS IN M51

The Toomres' (1972) test-particle simulations produced the tidal bridg e and tail structures in M51 with one companion passage. However, self -gravitating simulations with this orbit (Howard and Byrd 1990, Hernqu ist 1990) fail to produce the inner arm pattern. Subsequent single pas sage self-gravitating simulations (Hernquist 1990 and Toomre 1994) hav e produced inner arms but have had difficulty obtaining a close match to these and other features. An extended hydrogen ''far tail'' and clu mps north of the companion (e.g. Rots et al. 1990, Appleton et al. 198 6) are also difficult to create with; only one recent disk crossing. H oward and Byrd (1990) proposed that all these features could be explai ned by two disk crossings with the companion in a bound orbit. Here, w e more realistically simulate this hypothesis using a fully 3-D self-g ravitating code including collisionless stellar and collisional gas cl oud components. We quantitatively reproduce the morphology of the arms and outer hydrogen features as well has their doppler shift features