Annu. Rev. Astron. Astrophys. 1978. 16: 103-39 Copyright © 1978 by Annual Reviews. All rights reserved

5. CONCLUDING REMARKS

The ever-increasing refinement of observing methods has clearly produced a wealth of new and detailed kinematical information on disk galaxies in the last few years. A number of general features are appearing, superposed on the major pattern of circular rotation: large-scale disturbances in the inner regions are possibly associated with bars and oval distortions or with nuclear explosions; warping of the disks in the outer parts is not uncommon; the flatness of rotation curves at large radial distances has important consequences for models of the mass distribution.

The translation of these results into statements on the internal dynamics of galaxies is, however, not always clear. The goal of discovering the physical basis for the morphological classification schemes remains a distant one, apparently confused by a number of effects that disturb the pattern of central symmetry so common to the models. Indeed, when studied in detail each and every galaxy has unique properties that would seem to defy generalization; and yet no one can deny the ability of the various morphological classification schemes to put the great majority of galaxies into ordered sequences.

Although more detailed dynamical models of particular well-observed galaxies can obviously be made and would be of great value, it seems to us that the picture may also become clearer by taking a somewhat less-detailed look at a larger sample of galaxies covering a range of morphological types and luminosities. It would surely have been difficult to arrive for example at the Hubble sequence (e.g. Hubble 1926) with a sample of scarcely more than twenty or thirty galaxies, the number discussed in this review.

We can therefore make the safe, canonical statement that more data are needed. Experience shows that combined observations at radio and optical wavelengths on the same objects are particularly fruitful. In the future these measurements should be aimed at obtaining an ever increasing body of homogeneous observational data to provide information on the radial distributions of properties such as the optical and radio continuum emission, gaseous and total mass densities, and colors and abundances. After all, "The nature of the universe may eventually be rationalized by theoretical understanding, but it will not be discovered in the first place by pure thought" (King 1975).

ACKNOWLEDGMENTS

We are pleased to thank several of our colleagues in Groningen for their comments, and especially G.S. Shostak for a critical reading of an earlier version of this paper. We thank Mrs. R. Olde for her work on the manuscript. We are grateful to Drs. A. Bosma, J.M. van der Hulst, D.H. Rogstad, V.C. Rubin, and H.C.D. Visser for permission to reproduce their figures.