Annu. Rev. Astron. Astrophys. 1982. 20: 547-85 Copyright © 1982 by Annual Reviews. All rights reserved

EPILOGUE

The X-ray observations have helped to clarify and further our understanding of processes affecting galaxies, the intracluster medium, and the cluster as a whole. Analyses of the X-ray surface brightness have been used to trace massive halos around dominant galaxies in unevolved clusters and to define the cluster gravitational potential. Mapping the unseen virial mass with X-rays resulted in the discovery of the multiple-component "double" clusters, which have a bimodal mass distribution. Material has been observed as it is ram-pressure stripped from a hot corona around the M86 galaxy in Virgo. In the dynamically young cluster Abell 1367, such galactic coronae also have been observed. In both unevolved and evolved clusters, evidence for accretion flows onto central, dominant galaxies has been found. In addition, X-ray observations have permitted quantitative estimates of the fractions of young and evolved clusters, as well as new determinations of the total cluster mass.

However, while providing much new information, the observations have posed new problems that require additional theoretical and observational study. Spatially resolved X-ray spectroscopy is needed in many aspects of cluster research. This would enable more reliable studies of the cluster mass distribution and permit more detailed modeling of local galactic enhancements and central cooling flows. X-ray surveys of clusters at large redshift will permit the comparison of their luminosity function with that of clusters at the present epoch, and will allow a determination of evolutionary effects. The origin of the X-ray-emitting gas can be studied both by investigations of distant clusters and of the distribution and abundance of iron in more nearby systems.

A thorough understanding of the X-ray emission from clusters involves a wide variety of astrophysical problems. For example, the existence of an early generation of massive stellar systems would have major implications for the origin and heating of the intracluster gas. The origin of S0 galaxies ("nature or nurture") also has direct relevance to the source of the X-ray-emitting gas. Understanding of the formation and growth of cD galaxies may shed light on the development of the X-ray morphological classes. Numerical simulations of galaxy clusters and computations of the gas dynamics should increase knowledge of the structure and evolution of clusters.

"The present is very much the epoch of cluster formation," concluded Gunn & Gott (1972). The large number of clusters exhibiting an early evolutionary stage characterized by low X-ray luminosity, low galaxy concentrations, high spiral fraction, irregular morphology, and the relative rarity of evolved X-ray-luminous clusters like Coma and Abell 85 support this contention. The present variety of clusters provides a wealth of opportunity for observing the physical processes that occur during a cluster's dynamical collapse and evolution.

ACKNOWLEDGMENTS

We thank our friends for their patient and critical reading of this manuscript. In particular, we are grateful to R. Giacconi, H. Tananbaum, S. Murray, L. Van Speybroeck, J. Schwarz, D. Schwartz, S. Baum, P. Gorenstein, D. Harris, D. Fabricant, A. Lightman. A. Fabian, P. Nulsen, Y. Avni, J. Bechtold, A. Cavaliere, R. Mushotzky, R. White, and C. Canizares. We also acknowledge the unflagging efforts of K. Gilleece in preparing innumerable drafts. This work was supported under NAS8-30751.