Lensing constraints on the cores of massive dark matter halos

The statistics of wide-separation (6 " < <theta> < 15<double prime>) 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 proportional to r(-alpha) (1.0 less than or equal to alpha less than or equal to 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 similar to4% of the virial radius, f(core) < 0.023 (95% confidence level, <Lambda> cold dark matter). 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 Two-degree Field and Sloan Digital Sky Survey should detect wide-separation lenses and thus provide a measurement of the core mass fraction in massive dark matter halos.