Incomplete relaxation

(Left) Initial distribution of 6 galaxies in a compact group. An N-body code follows the evolution of this group. The subsequent collisions and mergers experience violent relaxation, and ultimately form a smooth elliptical-like galaxy with de Vaucouleurs surface brightness law (Right; notice the smaller scale). This final state doesn't evolve much more with time -- the potential is smooth and unchanging. Simulation is from Barnes 1989 [o-link].
A plot of the phase space distribution in r and Vr for the final configuration. Despite the smooth appearance of the final galaxy, there is considerable substructure in the phase space diagram. Clearly, relaxation has not been complete. Notice this is the principle behind searching for "streams" in the Milky Way halo, or the halos of nearby galaxies (streams in position-velocity space). A movie of a similar simulation (also by Josh Barnes) is here (10 Mb .mpg file).