Clusters: Gravitational lensing

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 below.




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.

High-redshift quasars are often lensed into multiple images by galaxies, three of which are shown above. They can be used for accurate measurements of various global cosmological parameters, provided that the mass distribution of the gravitational lens system is well constrained. Since the light travel time along the path to the various images is different, observing delayed versions of the same source event in its various images provides a direct way to measure the Hubble constant, independent of local calibrators. The ``cleanest" systems to do these are in the four-image systems as above. However, in many of these cases, it is obvious that the lensing effect is not due to a single galaxy, but contributions from the group or cluster it belongs to are significant.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. Our analysis of the system B1422+231 (the image on the right above) was recently published, and mentioned in the September 2003 issue of Natural History magazine.

We have also 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"). 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.


Analysis of systematic distortions of background galaxies (weak lensing) reveal significant structures in galaxy clusters that are not apparent from optical images. We are using XMM and Chandra observations to detect these dark subclusters (like in Abell 1722 above, from Dahle et al.), and investigate sites of suppressed star formation.

Researchers: Somak Raychaudhury, Rowan Temple, Habib Khosroshahi, Trevor Ponman