Dark matter: Gravitational lens studies

Abstract

Studies of dark matter using gravitational lenses are reviewed, with emphasis on recent maps of dark matter in galaxy clusters. Although there are several QSOs which are multiply imaged by isolated intervening galaxies, distant galaxies are 4 million times more numerous than QSOs and thus can sample a lens at multiple positions. To a surface brightness of 29 B mag arcsec−2 there are about 100 background galaxies per square arcmin, which is sufficient to map statistically the dark matter distribution in a foreground galaxy cluster. Automated pattern recognition software generates a 2‐d mass density map. There is evidence for a soft core of about 100 kpc radius for the dark matter in the rich clusters studied so far. These lens studies confirm the large mass in rich galaxy clusters which were implied by virial calculations using velocity dispersion. The dark matter distribution in a cluster is found to be similar to the distribution of the bright galaxies.