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 in these clusters 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 ρ ∝ r-1 and indistinguishable from recent cold dark matter simulations. Assuming that baryons make only a small difference to the profile outside ∼10 kpc, these results support the prediction of a universal inner profile for dark matter structures, independent of any parameterization of that profile.
- Dark matter
- Gravitational lensing