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About one fifth of all spiral disk galaxies include a ring-shaped pattern in the light distribution, and an additional one third appear to have broken or partial rings made up of spiral arms (pseudorings). These rings are a special problem in galaxy morphology with a direct bearing on the internal dynamics and evolution of disk galaxies. Morphological data have shown that rings are most often associated with bars or other common nonaxisymmetric perturbations, such as ovals. Kinematic and metric data have provided considerable evidence for intrinsic oval shapes and preferred alignments between ring major axes and bars. Photometric data have demonstrated that most rings are sites of current active star formation, and in some galaxies a ring is the only place where recent star formation is found. A few rings are sites of the most spectacular ``starbursts'' known in non-violently interacting galaxies.
Though a small fraction of observed rings may be due to collisions or mergers of galaxies, or to accretion of intergalactic gas, the vast majority of rings are probably simple resonance phenomena, caused by the actions of a rotating bar or other nonaxisymmetric disturbance on the motions of gas clouds in the disk. The evidence in support of this idea has accumulated steadily during the past 15 years, and our goal in this review is to bring together a large body of theoretical and observational results in one place. We shall see that rings are a natural consequence of barred galaxy dynamics, and that they are more easily understood than the bars and ovals which undoubtedly create them. However, there are interesting problems, such as the lack of any rings in some barred galaxies, the less common but by no means rare cases of rings in nonbarred galaxies, the role of mild tidal interactions, where the gas that fuels star formation in rings actually comes from, the existence of intrinsic bar/ring misalignment, and the simultaneous existence of different ring types of very different time-scales in the same galaxy. We will discuss these problems in some detail here, and indicate where the solutions may lie.
Keywords: Galaxies - structure; dynamics; evolution; rings; bars; simulations