Lensing Constraints on the Cores of Massive Dark Matter Halos

The statistics of wide-separation (6'' < theta < 15'') gravitational lensesconstrain the amount of mass in the cores of dark matter halos on group andcluster 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 lensesin the large Cosmic Lens All-Sky Survey yields an upper limit on the fractionof 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 thecold dark matter paradigm. While the halo profiles derived from numericalsimulations appear to be consistent with this upper limit, larger surveyscurrently underway such as the 2dF and SDSS should detect wide-separationlenses and thus provide a measurement of the core mass fraction in massive darkmatter halos.Comment: accepted for publication in ApJL; 5 pages, 2 figures, emulateap