Collisional Ring Galaxies

TL;DR: In this article, it was shown that rings and all collisions are overrepresented in deep HST fields, and Lavery et al. (1996) used this fact, and their relative rarity at low-redshift, to conclude that rings (and therefore presumably all collisions) are over-represented.

Abstract: Ring galaxies are believed to represent a special case of a collision between two galaxies, in which one of the galaxies impacts and passes through the center of another disk system (e.g. Lynds & Toomre 1976). Although rare, this kind of low orbital-angular-momentum collision leads to a recognizable structure, namely a luminous blue star-forming ring (Appleton & Marston 1997), which should be easily identifiable even at moderate redshift. Indeed, Lavery et al. (1996) have used this fact, and their relative rarity at low-redshift, to conclude that rings (and therefore presumably all collisions) are over-represented in deep HST fields.