|| © 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,
- What is the accurate baryon-fraction in clusters and groups of
galaxies, as a function of scale,
- What is the origin of the hot intracluster gas and its
- What is the evolution of clusters in the optical and in X-rays?
- What are the cosmological implications from studies of
- 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
- What is the mass, and mass distribution, in superclusters and on
large scales? Does mass follow light?
- What is in the "voids"?
- What is
- 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.
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.