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

TL;DR

This paper uses a perturbative method to reconstruct a complex gravitational lens, revealing an independent dark matter component likely caused by merging halos, providing new insights into dark halo structures at galaxy scales.

Contribution

The paper applies a perturbative reconstruction technique to a multi-galaxy lens, identifying a dark matter component that does not follow light, linked to halo merging.

Findings

Reconstruction of the lens's potential and density fields.

Detection of a dark component not following luminous matter.

Dark halo shape attributed to merging of cold dark matter halos.

Abstract

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.