Using weak-lensing dilution to improve measurements of the luminous and dark matter in A1689

The E/S0 sequence of a cluster defines a boundary redward of which a reliable weak-lensing signal can be obtained from background galaxies, uncontaminated by cluster members. For bluer colors, both background and cluster members are present, reducing the average distortion signal by the proportion of unlensed cluster members. In deep Subaru and HST/ACS images of A1689, the tangential distortion of galaxies with bluer colors falls rapidly toward the cluster center relative to lensing signal of the red background. We use this dilution effect to derive the cluster light profile and luminosity function to large radius, with the advantage that no subtraction of far-field background counts is required. The light profile declines smoothly to the limit of the data, r < 2 h^-1 Mpc, with a constant slope, d log (L)/d log (r) = -1.12 ± 0.06, unlike the lensing mass profile which steepens continuously with radius, so that M/L peaks at an intermediate radius, ≃100 h ^-1 kpc. A flatter behavior is found for the more physically meaningful ratio of dark matter to stellar matter when accounting for the color-mass relation of cluster members. The cluster luminosity function has a flat slope, α = -1.05 ± 0.05, independent of radius and with no faint upturn to M_i1 < -12. We establish that the very bluest objects are negligibly contaminated by the cluster [(V - i′)_AB < 0.2], because their distortion profile rises toward the center following the red background, but offset higher by ≃20%. This larger amplitude is consistent with the greater estimated depth of the faint blue galaxies, (z) ∼2 compared to (z) ∼0.85 for the red background, a purely geometric effect related to cosmological parameters. Finally, we improve on our earlier mass profile by combining both the red and blue background populations, clearly excluding low-concentration CDM profiles.