Gravitational lensing offers a new probe of Dark Matter: the light from distant galaxies is bent by the gravity of foreground halos, distorting the galaxy images. KICP members Erin Sheldon and Joshua Frieman and their collaborators on the SDSS team used the shapes of 8 million galaxies to show that galaxies trace the dark matter on large scales, consistent with the Universe containing 30% Dark Matter, 70% Dark Energy.

The Sloan Digital Sky Survey (SDSS), the most ambitious project to map the Universe ever undertaken, has measured images for tens of millions of galaxies and three-dimensional positions (using redshifts obtained from spectra of these galaxies) of about 500,000 objects to date. The project uses a dedicated 2.5-meter telescope in southern New Mexico.

One recent highlight of the project was the measurement of the halos of dark matter that surround luminous galaxies by KICP Fellow Erin Sheldon, former KICP graduate student David Johnston (now a postdoctoral fellow at Princeton) and KICP faculty member Josh Frieman, and their collaborators. This measurement used the relatively new technique of weak gravitational lensing: the shapes of about 8 million distant galaxies were measured and used to infer the mass distribution around 100,000 foreground galaxies. As the light from the distant galaxies passes by the foreground galaxies, it is bent by their gravitational fields, leading to a small distortion of the shapes of the background galaxies. Measuring that distortion allows one to make inferences about the halos of dark matter in which the foreground galaxies are enshrouded. A paper describing these results appeared in the May 2004 issue of the Astronomical Journal and at <a target='_blank' href='http://xxx.lanl.gov/abs/astro-ph/0312036'>Astrophysics abstracts</a>.

The graph shows the measurement of the galaxy-mass correlation function (points) from weak lensing, compared with the galaxy-galaxy correlation function (solid curve) from the Sloan Digital Sky Survey (upper panel). Lower panel shows the ratio of these two, which constrains the relation between the dark and luminous matter distributions as well as the mean density of the Universe.

Related Links:
KICP Members: Joshua A. Frieman; Erin S. Sheldon
Scientific projects: Sloan Digital Sky Survey (SDSS)