A picture of galaxy formation and evolution is beginning to emerge and become credible. It is not simple. Recent work shows that a large number of different processes can be important in any one galaxy (Fig. 49). Distinguishing between these is not easy. We are gradually discovering that we need to accept a more complex view of galaxies than we originally considered to be aesthetically pleasing (e.g., King 1977). This is not to say that we stop looking for simple generalizations. But it is important that we not mislead ourselves by trying to make things simpler than they are. I know of no more relevant statement than this one by Eiseley (1975):
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Figure 49. A partial morphological box (Zwicky 1957) of processes of galaxy formation and evolution. Three basic kinds of processes are distinguished, ones involving an initial collapse (perhaps in several stages), and subsequent evolutionary effects, which may be internally or externally driven. Some processes already found to be significant are listed, others probably remain to be discovered. All of them interact with star formation processes (center). Many of them also depend critically on the physics before galaxy formation, which is not illustrated. Our task is to distinguish the relative importance of the various processes in producing the galaxies that we see (and do not see). |
"The universe [may be] too frighteningly queer to be understood by minds like ours. It's not a popular view. One is supposed to fourish Occam's razor and reduce hypotheses about a complex world to human proportions. Certainly I try. Mostly I come out feeling that whatever else the universe may be, its so-called simplicity is a trick,.... I know we have learned a lot, but the scope is too vast for us. Every now and then if we look behind us, everything has changed. It isn't precisely that nature tricks us. We trick ourselves with our own ingenuity. I don't believe in simplicity."
At the same time, all of this underlying complexity often produces a remarkable beauty and regularity (Fig. 50). Eiseley's comment is a sobering lesson, but it does not diminish the enthusiasm that accompanies rapid progress. It is worth belaboring one more time the theme that much of this progress is made possible by recent technical advances. The present period is the only one we will ever live through in which the quantum efficiency of detectors went from low values to almost 100%. We are also opening up most of the electromagnetic spectrum for study. Such developments allow us to attack a variety of new and difficult physical problems. This is therefore an interesting time to be studying galaxies.
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Figure 50. NGC 5363 (upper) and NGC 5364, a pair of galaxies with very different morphological types. This illustration is from a IIIa-J + Wr2c plate taken with the Palomar 1.2 m Schmidt telescope. |
ACKNOWLEDGEMENTS
I am most grateful to the many people who sent preprints and who allowed me to quote results and reproduce figures before publication. They are too numerous to mention individually, but are identified in the figure captions and references. I am also grateful for helpful discussions with or comments on the manuscript from Drs. S. van den Bergh, J. Binney, N. Capaccioli, S. M. Fall, J. S. Gallagher, F. D. A. Hartwick, G. Illingworth, F. Schweizer (who also provided Fig. 18), and S. White. My observations of galaxy kinematics were obtained as a Visiting Astronomer at the Kitt Peak National Observatory and the Cerro Tololo Interamerican Observatory. It is a pleasure to thank Dr. G. Burbidge for the hospitality of the Kitt Peak Observatory during the extensive data reductions, Dr. P. Schechter for kindly making available his Fourier quotient program and Drs. G. Illingworth and D. McElroy for technical support. I also thank Dr. S. van den Bergh for his support of the efficient completion of the MS. The long typing job was competently shared by R. Haapala, M. Ker and B. Woodfield. I am grateful to David Duncan for his diligent and careful work to finish the many photographic slides and figures. R. Price kindly provided editorial help with the references. Finally, it is a particular pleasure to thank Mary Ker for her dedicated editorial help, without which this MS could not have been finished on schedule.