N-body simulations of interacting disc galaxies
  Disc galaxies can be substantially modified by close encounters and mergers, since their discs are very responsive components. Close interactions can be held responsible for the formation of bridges and tails, as well as for the formation of some bars, asymmetries and grand design spirals. Bound clumps can form in the tails, due to self-gravity, and could evolve to dwarf galaxies. Off-centerings and asymmetries in the central parts of barred galaxies can be made by off-centered and/or oblique impacts of sufficiently massive and compact companions. Similar impacts, but preferably centered, on non-barred galaxies can form ring galaxies. Companions on initially near-circular orbits can also cause changes to the target disc as they spiral gradually inwards. Low density companions are disrupted before reaching the center of the target and their debris form a thick disc. On the other hand most of the mass of the high density companions reaches the center, where it may form a bulge, thus entailing evolution along the Hubble sequence. Such companions thicken and expand the target disc and may also destroy bars in it. If their initial orbital plane is at an angle to the plane of the disc of the target, they can cause the latter to tilt substantially, depending on their mass and initial inclination. 
E. Athanassoula