Strong Lensing by a Node of the Cosmic Web. The Core of MACS
  J0717.5+3745 at z=0.55
  We present results of a strong-lensing analysis of MACS J0717.5+3745
(hereafter MACS J0717), an extremely X-ray luminous galaxy cluster at z=0.55.
Observations at different wavelengths reveal a complex and dynamically very
active cluster, whose core is connected to a large scale filament extended over
several Mpc. Using multi-passband imaging data obtained with the Hubble Space
Telescope Advanced Camera for Surveys (ACS), we identify 15 multiply imaged
systems across the full field of view of ACS, five of which we confirmed
spectroscopically in groundbased follow-up observations with the Keck
telescope. We use these multiply imaged systems to constrain a parametric model
of the mass distribution in the cluster core, employing a new parallelized
version of the Lenstool software. The main result is that the most probable
description of the mass distribution comprises four cluster-scale dark matter
haloes. The total mass distribution follows the light distribution but strongly
deviates from the distribution of the intra-cluster gas as traced by the X-ray
surface brightness. This confirms the complex morphology proposed by previous
studies. We interpret this segregation of collisional and collisionless matter
as strong evidence of multiple mergers and ongoing dynamical activity. MACS
J0717 thus constitutes one of the most disturbed clusters presently known and,
featuring a projected mass within the ACS field of view (R=150"=960 kpc) of
2.11 +- 0.23 x 10^{15} M_{sun}, the system is also one of the most massive
known.
