Lensing Constraints on the Cores of Massive Dark Matter Halos

The statistics of wide-separation (6'' < theta < 15'') gravitational lenses constrain the amount of mass in the cores of dark matter halos on group and cluster mass scales. For a family of halo models with a central cusp $\rho \propto r^{-alpha}$ (1.0 <= alpha <= 1.9), the lack of wide-separation lenses in the large Cosmic Lens All-Sky Survey yields an upper limit on the fraction of the halo mass that is contained within ~4% of the virial radius, f_{core} < 0.023 (95% confidence level, LCDM). This limit offers an important test of the cold dark matter paradigm. While the halo profiles derived from numerical simulations appear to be consistent with this upper limit, larger surveys currently underway such as the 2dF and SDSS should detect wide-separation lenses and thus provide a measurement of the core mass fraction in massive dark matter halos.