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.