Sparse reconstruction of the merging A520 cluster system
  Merging galaxy clusters present a unique opportunity to study the properties
of dark matter in an astrophysical context. These are rare and extreme cosmic
events in which the bulk of the baryonic matter becomes displaced from the dark
matter halos of the colliding subclusters. Since all mass bends light, weak
gravitational lensing is a primary tool to study the total mass distribution in
such systems. Combined with X-ray and optical analyses, mass maps of cluster
mergers reconstructed from weak-lensing observations have been used to
constrain the self-interaction cross-section of dark matter. The dynamically
complex Abell 520 (A520) cluster is an exceptional case, even among merging
systems: multi-wavelength observations have revealed a surprising high
mass-to-light concentration of dark mass, the interpretation of which is
difficult under the standard assumption of effectively collisionless dark
matter. We revisit A520 using a new sparsity-based mass-mapping algorithm to
independently assess the presence of the puzzling dark core. We obtain
high-resolution mass reconstructions from two separate galaxy shape catalogs
derived from Hubble Space Telescope observations of the system. Our mass maps
agree well overall with the results of previous studies, but we find important
differences. In particular, although we are able to identify the dark core at a
certain level in both data sets, it is at much lower significance than has been
reported before using the same data. As we cannot confirm the detection in our
analysis, we do not consider A520 as posing a significant challenge to the
collisionless dark matter scenario.
