Cluster gravitational lenses

Abstract

There are three primary theoretical challenges for the interpretation of cluster gravitational lenses. The first is the development of the statistics of are formation, for which some preliminary results were presented here (Bergmann, this volume). The second is the range and variation of models for arcs. While it is important to demonstrate that lensing can produce a particular image geometry, in the long run, the amount of extractable information depends on the range and variation of successful models. The third challenge is the development of robust, well characterized methods for the interpretation of arclets. Coupled to the theoretical challenge is the need to release more of the data on these systems. Among the observational answers theoreticians need are the following. How many arcs are there? What is the distribution of arc morphologies (broken arcs, single giant arcs, counter-arcs, “straight-arcs”,...)? Where do you not see arcs? For statistical studies it is just as important to know where arcs are not found as it is to know where they are found. Are optical rings lensed objects? Are there more complicated image geometries? For the development of arclet inversion schemes, we need published lists of arclets and their properties. As the field advances we can hope to answer many important questions about the properties of clusters and galaxies in clusters. Lensing already indicates that cluster core radii are smaller than generally expected, and they provide good confirmation of kinematic estimates for the mass to light ratio. With further advances in observations and interpretation, we should be able to directly map cluster potentials using the properties of arclets, determine the structure of galaxy halos in a cluster environment, limit the nature of the distributed matter in clusters, and estimate the redshift distribution of background galaxies.