We present a survey of mass profiles and mass-to-light ratios of eight typical galaxy clusters at a common redshift (z \sim 0.2). We use weak gravitational lensing as a probe because it is unique in avoiding any assumptions about the dynamical state of the clusters. To avoid bias toward the rare and spectacular clusters that are easy targets for lensing work, we selected an ensemble of much more common clusters with moderate X-ray luminosity. Although the survey is still in progress, two conclusions are emerging: (1) within a cluster, mass follows light very closely on the angular scales that we can measure, 0.2-2h^{-1} Mpc, and (2) there is a significant cluster-to-cluster scatter in mass-to-light (M/L) ratios despite uniformity of observing, reduction, and analysis procedures. We also derive an estimate of Omega based on extrapolation from the mass properties of these typical clusters. Finally, we discuss the discovery of other clusters in our fields through their lensing signal.
Extremely rich populations of globular clusters, numbering ten thousand or more, surround the central giant galaxies in rich clusters. I discuss some recent spectroscopic and photometric observations of these rich globular clusters populations. In very nearby galaxy clusters, the globular cluster velocities can be used to trace the transition from the galaxy potential to the larger cluster potential. Beyond c{z},approx,3000 \kms, the globulars are too faint for spectroscopic studies with even the largest telescopes and cease to be useful dynamical tracers. However, the spatial distribution and size of the globular population also contains information on the central mass distribution of the cluster and can be measured out to c{z}\,\gta\,}20,000 \kms. Overall, the data suggest that these rich globular cluster populations are comprised of normal, old globulars, the majority of which were assembled in the cluster centers before most of the stars in the cD galaxy were formed.
Tolman-Bondi-model and distances from the Tully-Fisher relation yield a distance to Virgo cluster 20-24 Mpc. Comparison with Virgo core galaxies (Federspielet al.) seems to favour 24 Mpc. High quality distances from the extragalactic Cepheid PL-relation support 21 Mpc, which is our preferred value (Ekholm et al., \cite{E00}) . The Virgo cluster mass is 1.5-2 times larger than the Virgo virial mass (Tully \& Shaya ). We find a steeper density profile for the gravitating matter (2.5-2.85) than for luminous matter (2.3).
We present the results of a dynamical analysis as well as the population content of three rich, X-ray luminous galaxy clusters, Abell222, Abell223 and Abell520, that are at intermediate redshifts.