`Cold' Galaxies versus `Hot' Galaxies:

Almost all of the observational evidence of galaxy collisions concerned disk galaxies, clearly because rotating disks are the most susceptible to long tidal features. Elliptical galaxies, whose structure is determined by high, anisotropic velocity distributions, would not be expected to develop thin interaction signatures. However, enhanced imaging in the 1980's showed that some of the previously assumed normal ellipticals displayed very faint, sharp-edged shells.

The N-body theorists went cheerfully back to their machines to show that shells in `hot' galaxies (ellipticals with high internal velocity dispersions) where due to the infall of `cold' disks (spirals with coherent, rotational phase space shapes).

So this is all very interesting, and the understanding of peculiar galaxies has the same influence on galaxy studies as abnormal psychology has on the study of human behavior (i.e. understand the perturbations from the norm to define what is normal). But are interactions common? And, if so, do they have a major impact on the appearance of galaxies today?