Perturbative reconstruction of a gravitational lens: when mass does not
  follow light
  The structure and potential of a complex gravitational lens is reconstructed
using the perturbative method presented in Alard 2007, MNRAS, 382L, 58; Alard
2008, MNRAS, 388, 375. This lens is composed of 6 galaxies belonging to a small
group. The lens inversion is reduced to the problem of reconstructing
non-degenerate quantities: the 2 fields of the perturbative theory of strong
gravitational lenses. Since in the perturbative theory the circular source
solution is analytical, the general properties of the perturbative solution can
be inferred directly from the data. As a consequence, the reconstruction of the
perturbative fields is not affected by degeneracy, and finding the best
solution is only a matter of numerical refinement. The local shape of the
potential and density of the lens are inferred from the perturbative solution,
revealing the existence of an independent dark component that does not follow
light. The most likely explanation is that the particular shape of the dark
halo is due to the merging of cold dark matter halos. This is a new result
illustrating the structure of dark halos at the scale of galaxies.
