Weak Lensing by Two [CLC][ITAL]z[/ITAL][/CLC] ∼ 0.8 Clusters of Galaxies

We report the detection of weak gravitational lensing of distant background galaxies by two rich, X-ray-luminous clusters of galaxies, MS 1137+66 at z =0.783 and RXJ 1716+67 at z = 0.813. We detect a shear signal in each cluster over the radial range 100≤ r ≤ 700 h −1 kpc. Assuming that the background galaxies lie in a sheet at z = 2, we measure a mass of 2.45 × 10 14 h −1 M ☉ and M / L V = 270 at 500 h −1 kpc radius for MS 1137+66 and a mass of 2.6 × 10 14 h −1 M ☉ and M / L V = 190 for RXJ 1716+67 at the same radius. Both the optical light maps and the weak lensing mass maps of RXJ 1716+67 show two spatially distinct subclusters, as well as a long filamentary structure extending out of the cluster to the northeast. In contrast, MS 1137+66 is an ultracompact massive cluster in both mass maps and optical light maps and contains a strongly lensed arc system in the cluster core. These data add to the growing number of massive clusters at z ≳ 0.8.