Weak Lensing Discovery and Tomography of a Cluster at z=0.68

We report the discovery, spectroscopic confirmation, and weak lensing tomography of a massive cluster of galaxies at z=0.68. The first shear-selected cluster from the Deep Lens Survey, a deep optical imaging survey of 28 square degrees, it dominates the weak lensing mass map of its 40-arcminute field. At more than twice the redshift of the only previous shear-selected cluster, it demonstrates that the redshift range of shear-selected cluster samples will be competitive with that of X-ray selected samples. Imaging data show an arc which is likely to be a strongly lensed background galaxy. The cluster's line-of-sight velocity dispersion is 980 km/s. A simple singular isothermal sphere model yields dynamical and weak lensing mass estimates (projected within a radius of 71 kpc) of $5.0 \pm 2.4$ and $5.1 \pm 2.0 \times 10^{13} M_\odot$ respectively; a rough lower limit from strong lensing is $1.8 \times 10^{13} M_\odot$. The tomography yields a lens redshift consistent with the cluster spectroscopic redshift. Mass maps and tomography of this quality or better will be produced by the Deep Lens Survey over an area a factor of sixty larger.