© CAMBRIDGE UNIVERSITY PRESS 1999

12. SOME UNSOLVED PROBLEMS

Considerable progress has been made over the last two decades in the study of clusters and superclusters of galaxies, as described in these lectures. However, many problems still remain open. I highlight some of the unsolved problems in this field that are likely to be solved in the coming decade. Currently planned large redshift surveys of galaxies and clusters such as the Sloan Digital Sky Survey and the 2dF survey, deep optical and X-ray surveys using HST, Keck, ROSAT, ASCA, and AXAF, among other, should allow a considerable increase in our understanding of the nature and evolution of these fundamental systems. At the same time, state of the art cosmological simulations to be available in the next decade (e.g., Ostriker, this book) should greatly enhance our ability to compare the observations with detailed expectations from various cosmologies and hopefully narrow down the correct cosmological model of our universe.

Here is a partial list of some of the interesting unsolved problems in the field of clusters and superclusters of galaxies.

Clusters of Galaxies

• What is the mass distribution and its extent in clusters of galaxies? Using gravitational lensing distortions, one can determine the mass density profile, _m(r), and total cluster mass, M(r), of clusters and compare it with the distribution of galaxies and gas for a large sample of clusters.

• Does mass follow light on these scales? If not, what is the bias factor as a function of scale, b(r)?

• What is the implied density parameter from clusters, _m(r)?

• What is the accurate baryon-fraction in clusters and groups of galaxies, as a function of scale, _b / (r)?

• What is the origin of the hot intracluster gas and its metallicity?

• What is the evolution of clusters in the optical and in X-rays?

• What are the cosmological implications from studies of clusters?

Superclusters

• What is, quantitatively, the morphology of superclusters and large-scale structure (superclusters, filaments, and void network)?

• What is the dependence of the superclustering properties on galaxy luminosity, surface brightness, type (E, S), and system (galaxies versus clusters)?

• What are the peculiar motions in superclusters and on large scales?

• What is the mass, and mass distribution, in superclusters and on large scales? Does mass follow light?

• What is in the "voids"?

• What is _m on large scales?

• What is the baryon fraction in superclusters?

• What is the time evolution of superclusters?

• What are the constraints made by the observed superclusters and large-scale structure on cosmology and galaxy formation models?

I expect that many of these questions will be addressed and possibly solved in the coming decade.

Acknowledgments

I thank the organizers of the Jerusalem Winter School 1995, J. P. Ostriker and A. Dekel, for an outstanding, productive, and fun school. The work by N. Bahcall and collaborators is supported by NSF grant AST93-15368.