Gravitational lensing detection of an extremely dense environment around
  a galaxy cluster
  Galaxy clusters form at the highest density nodes of the cosmic web. The
clustering of massive halos is enhanced relative to the general mass
distribution and matter beyond the virial region is strongly correlated to the
halo mass (halo bias). Clustering can be further enhanced depending on halo
properties other than mass (secondary bias). The questions of how much and why
the regions surrounding rich clusters are over-dense are still unanswered.
Here, we report the analysis of the environment bias in a sample of very
massive clusters, selected through the Sunyaev-Zel'dovich effect by the Planck
mission. We present the first detection of the correlated dark matter
associated to a single cluster, PSZ2 G099.86+58.45. The system is extremely
rare in the current paradigm of structure formation. The gravitational lensing
signal was traced up to 30 megaparsecs with high signal-to-noise ratio ~3.4.
The measured shear is very large and points at environment matter density in
notable excess of the cosmological mean. The boosting of the correlated dark
matter density around high mass halos can be very effective. Together with
ensemble studies of the large scale structure, lensing surveys can picture the
surroundings of single haloes.
