PROBING THE DYNAMICS OF CLUSTER-LENSES

We propose a new approach to study the dynamical implications of mass models of clusters for the velocity structure of galaxies in the core. Strong and weak lensing data are used to construct the total mass pro file of the cluster, which is used in conjunction with the optical gal axy data to solve in detail for the nature of galaxy orbits and the ve locity anisotropy in the central regions. We also examine other observ ationally and physically motivated mass models, specifically those obt ained from X-ray observations and N-body simulations. The aim of this analysis is to understand qualitatively the structure of the core, and to test some of the key assumptions of the standard picture of cluste r formation regarding relaxation, virialization and equilibrium. This technique is applied to the cluster Abell 2218, where we find evidence for an anisotropic core, which we interpret as indicating the existen ce of a dynamically disturbed central region. We fmd that the requirem ent of physically meaningful solutions for the velocity anisotropy pla ces stringent bounds on the slope of cluster density profiles in the i nner regions.