Gravitational Lensing by Galaxy Clusters

Priyamvada Natarajan

The presence of mass concentrations like massive galaxies or clusters causes light rays traveling from background sources (often galaxies or quasars) to be deflected, not unlike the bending effects caused by an optical lens. The amount of deflection produced is directly proportional to the `strength' of the lens, which is the mass in this case, as well as to the relative orientations of the lens and the object emitting the light. This HST image of the lensing cluster Abell 2218 (courtesy Kneib et al. 1996) at a redshift of 0.175 illustrates the dramatic `strong' lensing effects: arcs - the highly distorted structures that can be picked up by eye. Using the shapes of all the distorted background galaxies a high resolution mass map of this cluster has been constructed. Gravitational lensing provides incontrovertible evidence for the presence of copious amounts of dark matter in clusters of galaxies like this one.