The surprisingly steep mass profile of A1689, from a lensing analysis of subaru images

Subaru observations of A1689 (z = 0.183) are used to derive an accurate, model-independent mass profile for the entire cluster, r ≲ 2 Mpc h ^-1 by combining magnification bias and distortion measurements. The projected mass profile steepens quickly with increasing radius, falling away to zero at r ∼ 1.0 Mpc h^-1, well short of the anticipated virial radius. Our profile accurately matches the inner profile, r ≲ 200 kpc h^-1, derived from deep Hubble Space Telescope Advanced Camera for Surveys (ACS) images. The combined ACS and Subaru information is well fitted by a Navarro-Frenk-White profile with virial mass, (1.93±0.20) × 10¹⁵ M⊙, and surprisingly high concentration, c_vir = 13.7_-1.1^+1.4, significantly larger than theoretically expected (c_vir≃4), corresponding to a relatively steep overall profile. A slightly better fit is achieved with a steep power-law model, d log Σ(θ)/d log θ ≃ -3, with a core θ_Ci ≃ 1.′7 (r_c≃210 kpc h^-1), whereas an isothermal profile is strongly rejected. These results are based on a reliable sample of background galaxies selected to be redder than the cluster E/S0 sequence. By including the faint blue galaxy population, a much smaller distortion signal is found, demonstrating that blue cluster members significantly dilute the true signal for r ≲ 400 kpc h^-1. This contamination is likely to affect most weak lensing results to date.