Annu. Rev. Astron. Astrophys. 1985. 23: 147-168 Copyright © 1985 by Annual Reviews. All rights reserved

6. FINAL REMARKS

In several identifiable cases the outer parts of galaxies contain telltale evidence of recent interactions. For field ellipticals, in particular, such evidence is too common to justify the description "peculiar." In fact, only the outer rings discussed in Section 3, and possibly the dust and gas in some ellipticals and S0s, can be attributed to internal causes.

The test particle simulations by Toomre (70, 138-140), Quinn (88), Schwarz (102), and others reveal already most of the underlying physics, since in the outer parts of galaxies (the natural habitat of the shells and rings), self-gravity can be neglected. Thus, fast progress can be made by studying the fate of test particles falling into a rigid potential well.

We have relied on a completely different method to understand the shape and evolution of polar rings around S0s and possibly the dust and gas in ellipticals. Yet the two methods should be looked at simultaneously if one wants to model the case of a small disk galaxy falling into the potential well of a large galaxy. Obviously one cannot ignore the stellar component of the dwarf in the case of polar rings, or its gaseous component in the case of shells. Perhaps the viewing angles, or the infalling conditions (velocity, angle, impact parameter), are orthogonal in the two cases. Alternatively, the physical properties of the receiving galaxies, or even the infalling ones, may be different. An understanding of the X-ray emission in field ellipticals and S0s may shed further light on this.

One lingering question is, Why do shells and rings not form around all types of galaxies? One can argue that the oblate shape of the potential well likely to be prevalent in S0s and spirals is less favorable for efficient shell making, but some remnants of recent capture, even if these are all encircling, should be Identifiable around S0s. The available but very crude statistics (17, 57) show that field spirals have on average more close-in companions than do field S0s, and field S0S in turn have more than field ellipticals. Even if this is interpreted to suggest that fewer dwarfs have fallen into spirals than into ellipticals in their lifetime, some signs of capture should still be observable. Vague suggestions that captured, torn-apart dwarfs can serve to explain a variety of phenomena, such as proto-polar ring shapes around several galaxies, the presence of arcs around M81 (1) and M83 (152), asymmetries, as in M101 or NGC 1313 (7), or even warps (93), are no substitute for clear-cut demonstrations.

Thus we conclude that although individual mechanisms for the formation of shells and rings around galaxies are beginning to be understood, the global picture still needs to be elucidated. As discussed above, several basic questions on the fate of the stellar and gaseous components of a dwarf falling into a big galaxy remain unclear, while a better understanding of the capture process and settling times of polar rings could shed some light on the shapes of galaxies. Other interesting problems, such as the formation of stellar outer rings, the, orientation of outer rings in barred galaxies, and the formation of ripples deep in the' potential well of an elliptical, are still unanswered. On the observational side, more effort is also warranted. In particular, the subject of shells is very new and lacks basic information, both spectroscopic and photometric. Finally, since our knowledge of polar rings and ring galaxies is based on very few well-studied cases, an attempt should be made to construct a larger and more representative sample.

Acknowledgments

We wish to thank all our colleagues who helped us with preprints and with discussions. In particular, we thank Drs. Carter, Combes, Dennefeld, Dupraz, Fort, Gerhard, Habe, Ikeuchi, Madore, Quinn, Sadler, and Steiman-Cameron for sending us material in advance of publication.