Cluster Lensing: Arcs, Arclets and Shear

Clusters of galaxies are very massive, and their gravitational fields will deflect the light from background galaxies, producing deflections, distortions and multiple images. The diagram below shows this process.

Note that different radii in the cluster have vastly different focal lengths, so the images that are produced are very distorted. Background objects that are exactly on the line-of-sight from us toward the cluster produce ring-shaped images known as Einstein rings.

This animation shows the view we would have looking at the sky through a galaxy cluster with two mass concentrations (the pink shading indicates the projected mass density) which is lensing a background field of faint blue galaxies. In actual observations the transverse velocities are so small that there are no observable changes over time, but I have caused the background galaxy field to slide horizontally in order to show more examples of the kind of highly distorted and magnified images that are possible. Inside the Einstein ring radius one has radial arcs, and right in the center of the cluster there are demagnified images. The highest magnifications occur at the Einstein ring radius, giving the large tangential arcs. Outside the Einstein ring radius there is a systematic pattern of shear leading to tangential arclets. The magnitude of this shear pattern can be used to determine the mass contained within the cluster. Abell 2218 and CL2244-02 are clusters with tangential arcs.

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