The cluster lens ACO 1703: redshift contrast and the inner profile
  ACO 1703 is a cluster recently found to have a variety of strongly lensed objects: there is a quintuply-imaged system at z=0.888 and several other lensed objects from z=2.2 to 3.0 (the cluster itself is at z=0.28). It is not difficult to model the lens, as previous work has already done. However, lens models are generically non-unique. We generate ensembles of models to explore the non-uniqueness. When the full range of source redshifts is included, all models are close to \rho \propto r^{-1} out to 200 kpc. But if the quint is omitted, both shallower and steeper models (e.g., \rho \propto r^{-2}) are possible. The reason is that the redshift contrast between the quint and the other sources gives a good measurement of the enclosed mass at two different radii, thus providing a good estimate of the mass profile in between. This result supports universal profiles and explains why single-model approaches can give conflicting results. The mass map itself is elongated in the NW-SE direction, like the galaxy distribution. An overdensity in both mass and light is also apparent to the SE, which suggests meso-structure. 
Prasenjit Saha and Justin I. Read