Gravitational Optics Studies of Dark Matter Halos

One of the best methods that we have for exploring the distribution of dark matter throughout the universe is to measure its gravitational influence on the observed shapes of background galaxies. On linear scales of ∼ 3 – 300 kpc, when the dark matter distribution is dominated by individual galaxy halos, the investigation is best performed statistically by correlating the orientations of the source galaxies with the position angles of the foreground lens galaxies. Recent observational programs have shown that the large-scale mass to light ratio in red bandpasses is ∼ 100 solar units, and is apparently independent of galaxy type. The prospects for improving the precision of these measurements and for extending them to larger length scales over the next five years are good.