Two strong-lensing clusters confront universal dark-matter profiles

In the galaxy clusters SSDS J1004+411 and ACO 1689, a large number of multiply-imaged background objects have recently been observed. We use these data to map the projected mass distribution in the inner regions of these clusters. The source redshifts span a large range, which eliminates the degeneracies that plague nearly all lensing work. As a result the mass maps are very well-constrained, despite very weak prior assumptions. ACO 1689 lenses so many objects that we can afford to map it twice using disjoint sets of images, thus verifying our internal error estimates. We then deproject the mass maps (pretending for this purpose that they are spherical) and obtain inner profiles consistent with rho propto r^{-1} and indistinguishable from recent cold dark matter simulations. Assuming that baryons make only a small difference to the profile outside sim 10 kpc, these results support the prediction of a universal inner profile for dark matter structures, independent of any parametrization of that profile.