Annu. Rev. Astron. Astrophys. 2004. 42: 603-683 Copyright © 2004 by Annual Reviews. All rights reserved

9.2. Perspective

Secular evolution provides a new collection of physical processes that we need to take into account when we try to understand galaxies. Doing so has already led to significant progress. Thirty years ago, Hubble classification was in active and successful use. However, we also knew about a long list of commonly observed, regular features in disk galaxies, including lenses, boxy bulges, nuclear bars, and nuclear star clusters, that were not understood and that were not included in the classification schemes. In addition, we knew about uniquely peculiar galaxies (e, g., Arp 1966) that were completely outside the classification process. Now, almost all of the common features and peculiar galaxies have candidate explanations within one of two paradigms of galaxy evolution that originated in the late 1970s. The peculiar objects have turned out mostly to be interacting and merging galaxies. And many of the previously unexplained but common features of disk galaxies now are fundamental to our growing realization that galaxies continue to evolve secularly after the spectacular fireworks of galaxy mergers, dissipative collapse, and their attendant nuclear activity have died down.

ACKNOWLEDGMENTS

We are indebted to Ron Buta and Jerry Sellwood, to scientific editors Geoffrey Burbidge and Allan Sandage, and to the Nuker team, especially Sandy Faber and Scott Tremaine, for penetrating comments on the draft that resulted in important improvements in the final paper. We thank Ralf Bender, Martin Bureau, Andi Burkert, Leo Blitz, Ken Freeman, Sheila Kannappan, Mike Rich, and Guy Worthey for helpful discussions or for permission to quote results before publication. We are exceedingly grateful to Ron Buta and Marcello Carollo for making available many of the digital images used in the construction of figures. Additional figures were kindly made available by Lia Athanassoula, Martin Bureau, Ortwin Gerhard, and Linda Sparke. The color image of NGC 1326 in Figure 8 was constructed for this paper by Zolt Levay of STScI from UBVRI and H, HST PC images provided by Ron Buta. We are most grateful to Mary Kormendy for extensive editorial help. Also, we thank Mark Cornell for technical support and for permission to quote results before publication. This paper has made extensive use of the NASA/IPAC Extragalactic Database (NED), which is operated by JPL and Caltech under contract with NASA.

Approximately 50 references were removed from the published version because of editorial pressure to shorten this paper. We regret this, and we include these references here.