Strong and Weak Lensing by Galaxy Clusters

In some parts of the Universe, hundreds of galaxies are bound by their mutual gravity into enormous structures known as clusters. The lensing properties of galaxy clusters differ from those of individual galaxies for two reasons: a cluster is a hundred to a thousand times more massive than a galaxy, and the inner density profile of a cluster is shallower than isothermal. The large mass creates a non-negligible probability that two or more sources will be lensed by a single cluster, while the shallow density profile allows the creation of radial arcs. Section 7.1 extends our previous discussion of strong lensing to clusters. Moreover, since they are so massive clusters can produced distortions well beyond the strong lensing region. These distortions are too small to be detected individually, but they can be extracted from statistical analysis of thousands of galaxies. Section 7.2 introduces the theory of weak lensing and describes how measurements of shape distortions can be used to constrain lens mass distributions. Section 7.3 then discusses applications of both strong and weak lensing by clusters.