Toomre's early simulation of a cold disk falling into a potential, and comparison with NGC 3923.
Peter Quinn's simulation of shell formation
Simulation by Stelios Kazantzidis of a cold spiral disk falling into an elliptical galaxy potential (elliptical galaxy not shown). View the movie (37Mb) here: [movie].
The radius vs radial velocity phase space for Quinn's simulation at time 18. Notice how the shells show up much more clearly in this phase space than in projection on the sky.