A Weak-Lensing Mass Reconstruction of the Large-Scale Filament Feeding
  the Massive Galaxy Cluster MACSJ0717.5+3745
  We report the first weak-lensing detection of a large-scale filament
funneling matter onto the core of the massive galaxy cluster MACSJ0717.5+3745.
Our analysis is based on a mosaic of 18 multi-passband images obtained with ACS
aboard the HST, covering an area of \sim 10x20 arcmin^2. We use a weak-lensing
pipeline developed for the COSMOS survey, modified for the analysis of galaxy
clusters, to produce a weak-lensing catalogue. A mass map is then computed by
applying a weak-gravitational-lensing multi-scale reconstruction technique
designed to describe irregular mass distributions such as the one investigated
here. We test the resulting mass map by comparing the mass distribution
inferred for the cluster core with the one derived from strong-lensing
constraints and find excellent agreement. The filament is detected within the 3
sigma detection contour of the lensing mass reconstruction, and underlines the
importance of filaments for theoretical and numerical models of the mass
distribution in the Cosmic Web. We measure the filament's projected length as
\sim 4.5 h_{74}^{-1} Mpc, and its mean density as (2.92 \pm 0.66)10^8 h_{74}
M_{\odot} kpc^{-2}. Combined with the redshift distribution of galaxies
obtained after an extensive spectroscopic follow-up in the area, we can rule
out any projection effect resulting from the chance alignment on the sky of
unrelated galaxy group-scale structures. Assuming plausible constraints
concerning the structure's geometry based on its galaxy velocity field, we
construct a 3D model of the large-scale filament. Within this framework, we
derive the three-dimensional length of the filament to be 18 h_{74}^{-1} Mpc,
and a deprojected density in terms of the critical density of the Universe of
(206 \pm 46) \rho_{crit}, a value that lies at the very high end of the range
predicted by numerical simulations.
