|Annu. Rev. Astron. Astrophys. 1985. 23:
Copyright © 1985 by Annual Reviews. All rights reserved
The outer parts of galaxies often exhibit distinct surface brightness enhancements, either encircling the galaxy completely (rings) or only partially (arcs or shells). Such a phenomenological description can encompass dynamically very different situations, each of which must be understood if a given object is to be interpreted correctly. Indeed, confusions and misclassifications have been all too frequent.
Initially rings and shells around galaxies were ascribed to some internal mechanism, such as ejection, explosions, or magnetic forces. Yet as dynamical models of galaxies became more concrete and numerical experiments were performed, theories in which the interaction of a galaxy with its neighbors plays a key role came to the forefront. It has become increasingly evident that it is impossible to treat galaxies as isolated "island universes," since some of their characteristics are due to environmental influences.
The demonstration that gravity alone can produce shapes like bridges and tails (140) or ring galaxies (70, 133) was quickly followed by the realization that enough friction may exist between galaxies in close passages that they may merge on relatively short time scales (138). Thus sprang up the ideas of cannibalism among galaxies (85, 86), which is important in understanding the growth of cD galaxies at the expense of their neighbors, and of merging, candidate explanation for the formation of elliptical galaxies in the field (138) and the feeding of the "monster" in the nucleus of active galaxies (48).
Naturally the big events, such as those leading to the formation of ring galaxies, are interspersed with a whole set of smaller ones. In particular, small galaxies in a group are prone to falling into their bigger neighbors without affecting the latter very much. After the event, the big-galaxy is adorned with some frills, i.e. ellipticals with shells, S0s with polar rings, etc. We review here the properties of these types of galaxies, insofar as their outskirts have identifiable shells or rings. Spirals and S0s with outer rings fit a similar morphological description and are discussed here too, even though (as we shall see) their origin is quite different.