An Upper Limit on ΩmUsing Lensed Arcs

We use current observations on the number statistics of gravitationally lensed optical arcs toward galaxy clusters to derive an upper limit on the cosmological mass density of the universe. The gravitational lensing statistics from foreground clusters combine properties of both cluster evolution, which is sensitive to the matter density, and volume change, which is sensitive to the cosmological constant. The uncertainties associated with the predicted number of lensing events, however, currently do not allow one to distinguish between flat and open cosmological models with and without a cosmological constant. Still, after accounting for known errors, and assuming that clusters in general have dark matter core radii of the order ~35 h -1 kpc, we find that the cosmological mass density, Ω m , is less than 0.56 at the 95% confidence. Such a dark matter core radius is consistent with cluster potentials determined recently by detailed numerical inversions of strong and weak lensing imaging data. If no core radius is present, the upper limit on Ω m increases to 0.62 (95% confidence level). The estimated upper limit on Ω m is consistent with various cosmological probes that suggest a low matter density for the universe.