Computer Simulations
In the last decade there has been enormous interest in
computer simulation of galactic collisions. The physics
involved has been known since the time of Newton,
but only with modern computers have we been able to
produce results. One group working on this problem
http://research.amnh.org/~summers/mihos/mihos.html
where I picked up the following stills.
(See also http://zebu.uoregon.edu/movie.html)