Annu. Rev. Astron. Astrophys. 1992. 30: 705-742 Copyright © 1992 by Annual Reviews. All rights reserved

1. INTRODUCTION

Interacting galaxies encompass a tremendous range of phenomena - indeed, if events during galaxy formation are counted, there are probably few galaxies which were not shaped by interactions or even outright mergers (e.g. Toomre 1977; Efstathiou 1990). In order to keep our subject from completely overrunning the bounds of a single article, we focus on the dynamics of interacting galaxies as observed in our own epoch. Even within this focus, there are many interrelated topics - including star formation, nuclear activity, and galactic morphology - which continually tempt reviewers to depart on inviting tangents. While exploring some of these byways, this article makes no pretense at a complete coverage of the subject. A more observationally oriented review of galactic collisions has been presented by Schweizer (1986), while earlier numerical work has been summarized by White (1983b). Galactic interactions have featured prominently in recent conferences - the proceedings edited by Wielen (1990) and by Sulentic et al. (1990) together present a wider range of viewpoints on this subject than we could hope to encompass here. It is fortunate that excellent reviews of some related topics have recently appeared. In particular, the reader is referred to Sellwood (1987) for simulation techniques, to Soifer et al. (1987) for infrared-luminous galaxies, to Kormendy & Djorgovski (1989) and to de Zeeuw & Franx (1991) respectively for the observations and dynamics of elliptical galaxies, and to Osterbrock (1991) for the observational status of active galaxies.

Figure 1 presents a few well-known interacting systems, arranged roughly in order of increasing violence. M 51's dramatic spiral is very likely of tidal origin, but how secure would this inference be without the other signs of tidal damage to both galaxies? Next, we see gas being transferred from NCG 3808 to its smaller, spindle-like companion, where it may eventually form a polar ring. The ``Cartwheel'', on the other hand, is evidently the result of a direct hit by one of this galaxy's two small companions. NGC 4038 / 9, the ``Antennae'', originated in a close encounter of two comparable disk galaxies; several 10⁸ yr later extensive tidal tails have developed and the galaxies have apparently returned for a second passage. Two intertwined disk galaxies evidently make up NCG 520, which was once classified as a single irregular galaxy; CO observations indicate the presence of a substantial amount of molecular gas coincident with the dust lane crossing the face of this object. Finally, NGC 7252 is a merger remnant ~ 10⁹ yr old; it exhibits a relaxed body with a de Vaucouleurs' luminosity profile, a messy, irregular envelope, and a pair of extended tidal tails as a legacy of the two disk systems destroyed in its making.

Figure 1. A child's garden of galactic collisions (Barnes et al. 1991). Top row shows galaxies interacting with smaller companions; on the left is the ``grand design'' spiral M 51, in the middle is NGC 3808, and on the right is the ``Cartwheel'' galaxy. Bottom row shows more violent interactions; on the left are the ``Antennae'', in the middle is NGC 520, and on the right is NGC 7252.

The outline of this review is as follows. Numerical methods for modeling galactic interactions are discussed in Section 2. Next, Section 3 describes the signatures written on interacting galaxies by tidal forces. Events which add mass to a galaxy are the focus of Section 4, and major mergers between systems of comparable mass are discussed in Section 5. The many forms of activity triggered or induced by galactic interactions are the subject of Section 6. Finally, Section 7 discusses these galaxies' return to normality and related cosmological issues, and Section 8 lists some outstanding questions.