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

8. FURTHER QUESTIONS

Most astronomers would agree that galaxies interact and that gravity plays the leading role in the dynamics of such encounters. At the present, however, there is considerable controversy over the significance of interactions in shaping galaxies. The point of view adopted in this article is that interactions provide important examples of galaxy transformations.

Numerous questions remain: How much satellite accretion can disk galaxies stand? How much has occurred? What signatures exist in our galaxy? What are realistic parameters for merger progenitors and remnants? What range of remnant dynamics and orbital structure are permitted by the merger process? What distributions of axial ratios, minor-axis rotation, and non-elliptical isophote shapes are expected of mergers? Does long-term dynamical evolution play a role? How does gas lose angular momentum during interactions and mergers? How will a multiphase ISM behave? What effects will star formation have? What photometric, kinematic, and enrichment signatures of starburst events should we look for in old merger remnants? What controls the rate at which a monster is fed? What are the relative contributions of starbursts and nonthermal sources to IRAS galaxies? To powering ``superwinds''? Can ``buried quasars'' break through? And continue to shine after? Why were QSOs more common at z 2? Why so much brighter? Do a few, some, or most E galaxies have central black holes? What is the origin of the density-morphology relationship? How often are compact groups formed in evolving loose groups? How much merging took place in the evolution of rich clusters? How much substructure survives in regular clusters? Where do cD galaxies come from? How does the merger rate change over time? Can galaxies long survive the merging of their halos? What is the relation between bulges and ellipticals? What determines the specific frequency of globular clusters? ACKNOWLEDGEMENTS

We are especially grateful to John Kormendy, François Schweizer, Alan Stockton, and Alar Toomre for their generous encouragement and for comments on the manuscript. We also thank Halton Arp for the photographs of M 51, NGC 3808, and NGC 520, Victor Blanco and CTIO for the photograph of the Cartwheel, and François Schweizer for the photographs of NGC 4038 / 9 and NGC 7252 and for composing Figure 1. J.B. thanks the Canadian Institute for Theoretical Astrophysics for their hospitality and support during the initial work on this article. This work was supported in part by the Alfred P. Sloan Foundation, NASA Theory Grant NAGW-2422, Grants CS 79-90 and CS 47-91 from the California Space Institute, and NSF Grant AST 90-18526.