Measurement of the Mass Profile of Abell 1689

In this Letter we present calibrated mass and light profiles of the rich cluster of galaxies Abell 1689 out to 1 h-1 Mpc from the center. The high surface density of faint blue galaxies at high redshift, selected by their low surface brightness, is a unique tool for mapping the projected mass distribution of foreground mass concentrations. The systematic gravitational lens distortions of 104 of these background galaxies in 15' fields reveal detailed mass profiles for intervening clusters of galaxies and are a direct measure of the growth of mass inhomogeneity. The mass is measured directly, avoiding uncertainties encountered in velocity or X-ray derived mass estimates. Mass in the rich cluster Abell 1689 follows smoothed light, outside 100 h1 kpc, with a rest frame V-band mass-to-light ratio of 400±60 h-1 (M/Lv)sun. Near the cluster center, mass appears to be more smoothly distributed than light. From 200 h-1 kpc to 1 h-1 Mpc radius the surface density follows a steeper than isothermal profile. Comparing with preliminary high-resolution N-body clustering simulations for various cosmogonies on these scales, these data are incompatible with hot dark matter, are a poor fit to most mixed dark matter models, and favor open or λ > 0 cold dark matter. Substructure is seen in both the mass and the light, but detailed correspondence is erased on scales less than 100 h-1 kpc.