Weak Lensing by Two z~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 the background galaxies lie in a sheet at z=2, we measure a mass of 2.45*10^14 h^-1 Msolar and M/Lv = 270 at 500 h^-1 kpc radius for MS 1137+66, and a mass of 2.6*10^14 h^-1 Msolar and M/Lv = 190 for RXJ 1716+67 at the same radius. Both the optical lightmaps and weak lensing massmaps of RXJ 1716+67 show two spatially distinct sub-clusters, as well as a long filamentary structure extending out of the cluster to the northeast. In contrast, MS 1137+66 is an ultra-compact massive cluster in both massmaps and optical lightmaps, 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. 
D. Clowe, N. Kaiser, G. Luppino, J. P. Henry and I. M. Gioia