Clusters of galaxies: Gravitational lensing

In this image there are five blue, loop-shaped objects that actually are multiple images of the same galaxy. They have been multiply imaged by the dark matter in the cluster of yellow, elliptical and spiral galaxies - called CL0024+1654 - near the image's centre. This effect, known as gravitational lensing, is produced by the cluster's strong gravitational field that bends light to magnify, brighten and distort the image of a more distant object. The degree of distortion and number of copies depends on the alignment between the foreground cluster and the more distant galaxy, which is behind the cluster. The distribution of matter in the cluster of galaxies can be studied by the appearance and position of these images.

This image was taken with the Wide Field Planetary Camera-2 on board the Hubble Space Telescope-- more information about this and other images can be found here and here.

The light from distant galaxies and quasars is affected by the gravitational field of the intervening matter between us and the source, resulting in magnified or multiple images. To probe the distribution of matter in the Universe, which is overwhelmingly dark, one needs to directly map this gravitational field. The study of gravitational lensing therefore recently has become one of the most valued tools in surveying the universe and understanding its constituents and its evolutionary history. The most direct way of measuring the distribution of matter in a cluster of galaxies is to study gravitationally lensed images of galaxies lying behind the cluster, like in the image above.

We have developed an algorithm for the reconstruction of the distribution of matter in a cluster of galaxies from the observable distortion of background galaxies (``weak lensing"), together with colleagues at IUCAA. In this method, called the Lens Mapping Algorithm, from the measured distortion (``shear") of the images, the mass distribution is directly derived. This is unlike other methods, where the convergence is first obtained. We have shown that the strength of this method is that for finite fields, the usual mass-sheet degeneracy and other boundary problems can be eliminated by an iterative scheme.

Four-image lens systems This shows two quadruple-image gravitational lens systems from the CASTLES database. The dominant lens is the central galaxy seen in these near-infrared images taken by the Hubble space telescope. The asymmetric position of the images about the galaxy, however, is due to the lensing effect of the cluster that the galaxy belongs to.

We are carrying out a survey of x-ray detections of clusters and groups of galaxies that are required by gravitational lens models of multiply-imaged quasars but are not seen by optical means.