Perturbative reconstruction of a gravitational lens: when mass does not follow light

Aims. The structure and potential of a complex gravitational lens is reconstructed using the perturbative method presented in Alard (2007, MNRAS, 382, L58; 2008, MNRAS, 388, 375). This lens is composed of 6 galaxies belonging to a small group. Methods. 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. Results. 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. Conclusions. 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 on the scale of galaxies.