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 fifteen 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 z _ { l } = 0.30 \pm 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 .