Discovery of a Galaxy Cluster via Weak Lensing

We report the discovery of a cluster of galaxies via its weak gravitational lensing effect on background galaxies, the first spectroscopically confirmed cluster to be discovered through its gravitational effects rather than by its electromagnetic radiation. This fundamentally different selection mechanism promises to yield mass-selected, rather than baryon- or photon-selected, samples of these important cosmological probes. We have confirmed this cluster with spectroscopic redshifts of 15 members at z = 0.276, with a velocity dispersion of 615 km s-1. We use the tangential shear as a function of source photometric redshift to estimate the lens redshift independently and find zl = 0.30 ± 0.08. The good agreement with the spectroscopy indicates that the redshift evolution of the mass function may be measurable from the imaging data alone in shear-selected surveys.