Contents

1. Introduction

2. Historical Overview

2.1 Inner Ring Structures

2.2 Outer Ring Structures

2.3 Nuclear Ring Structures

3. Comparison of Ring Classifications

4. Recent Revisions to the Classification

4.1 Outer Ring and Pseudoring Morphologies

4.2 Inner, Outer, and Nuclear Lenses

5. Confusion of Ring Types in Nonbarred Galaxies

6. Summary of Notation for Ring and Lens Types

7. Rings and Lenses as Distinct Components of Barred Galaxy Structure

8. Rings and Nuclear Activity

9. Observations of Galactic Rings

9.1 Frequency

9.2 Linear and Relative Diameters

9.3 Intrinsic Shapes and Orientations

9.4 Photometric Properties

9.5 HII Regions in Ringed Galaxies

9.6 Neutral Hydrogen Emission from Ringed Galaxies

9.7 CO, IR, and Radio Continuum Emission from Ringed Galaxies

9.8 Kinematic Studies

10. Theories of Ring Formation

10.1 Some Historical Points

10.2 Fundamental Clues from Observations

11. Dynamics of Bars

11.1 Bar Formation

11.2 Orbits in a Barred Galaxy

11.3 Bars in Early- and Late-Type Galaxies

11.4 Pattern Speed Determination

11.5 Angular Momentum Transport

12. Gas Flows in Barred Galaxies

12.1 Hydrodynamics Modeling

12.2 Formation of Rings

12.3 Role of Viscosity

12.4 Fueling the Nucleus

13. Self-Gravitating Hydrodynamics

13.1 Stability

13.2 Self-Gravitating Rings and Nuclear Spirals

14. Problems with Ring Formation Theory

14.1 Time-scales and the Coexistence of Rings

14.2 Bar-Ring Misalignment

15. Rings in Nonbarred Galaxies

15.1 Bar Destruction, Lens, and Oval Formation

15.2 Evolution Along the Hubble Sequence

16. Interaction Between Several Waves

16.1 Nuclear Bars Inside Nuclear Rings

17. Tidal Interactions and Rings

17.1 The Leading Arm Ringed Galaxy NGC 4622

17.2 The One-Armed Outer-Ringed Galaxy NGC 4378

17.3 Accretion Rings

18. Single-Ringed, Multi-Armed Galaxies

19. Conclusions

References