DR. MATHILDE JAUZAC

MACSJ0717.5+3745 is an X-ray luminous cluster, at a redshift of z ~0.55. Previous analysis of the galaxy distribution in its surrounding field revealed the presence of an elongated structure which was thought to be a large-scale filament. By making a weak gravitational lensing analysis of this cluster and its outskirts, we were able to report the weak-lensing detection of a large-scale filament which funnels the matter into the core of the cluster. Our analysis is based on a mosaic of 18 HST.ACS maps, i.e. an area of ~10x20 arcmin2. To test the consistency of our weak lensing analysis, we first compared our results with the strong lensing analysis of Limousin et al. (2012, A&A) of the cluster core. The weak and strong lensing density profiles of the cluster core show a really good agreement. In terms of mass integrated in a radius of 500 kpc (given the same center), the strong lensing gives 1.06±0.03 1e15 Msun, while the weak lensing gives 1.04±0.08 1e15 Msun. The excellent agreement between both values and density profiles confirms the strength of our weak lensing analysis. Our analysis detects the MACSJ0717 filament 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. By complementing our imaging data with extensive spectroscopy of galaxies in the area, and applying 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 a three-dimensional length of the filament to be 18 Mpc. The filament's deprojected density in terms of the critical density of the Universe is measured as 206±46, a value that lies at the very high end of the range predicted by numerical simulations.