Clusters as Gravitational Lenses

Abstract

This paper reviews the present status of our understanding of gravitational lensing by clusters of galaxies. A spectacular manifestation of this phenomenon is the formation of highly distorted images of background sources (galaxies) known as gravitational arcs, but also the fonnation of more numerous small distorted images, the so-called arclets. Although this topic is still a recent one, some original new theoretical approaches have been developed and a lot of observations are perfonned on the largest telescopes through international collaborations, from which a large number of interesting results have already emerged. The redshift determination of more than 6 large arcs opens a new window in the study of high redshift field galaxies. The modeling of some of these arcs reinforces the idea that most of the dark matter in clusters is distributed in a smooth component outside the galaxies. The detection of more than 50 arclets in some rich clusters will enable one to better probe the mass content and the distribution of dark matter in clusters, as well as indications on the redshift distribution of the background galaxies.