Weak‐Lensing Discovery and Tomography of a Cluster atz= 0.68

We report the weak-lensing discovery, spectroscopic confirmation, and weak-lensing tomography of a massive cluster of galaxies at z = 0. 68, demonstrating that shear selection of clusters works at redshifts high enough to be cosmologically interesting. The mass estimate from weak lensing, 11. 1 ± 2. 8 × 10^ (14) (r/Mpc) M☉ within projected radius r, agrees with that derived from the spectroscopy (σᵥ= 980 km s^ (-1) ) and with the position of an arc that is likely to be a strongly lensed background galaxy. The redshift estimate from weak-lensing tomography is consistent with the spectroscopy, demonstrating the feasibility of baryon-unbiased mass surveys. This tomographic technique will be able to roughly identify the redshifts of any dark clusters that may appear in shear-selected samples up to z ~ 1.