Strong‐Lensing Analysis of A1689 from Deep Advanced Camera Images

Abstract

We analyze deep multicolor Advanced Camera images of the largest known gravitational lens, A1689. Radial and tangential arcs delineate the critical curves in unprecedented detail, and many small counterimages are found near the center of mass. We construct a flexible light deflection field to predict the appearance and positions of counterimages. The model is refined as new counterimages are identified and incorporated to improve the model, yielding a total of 106 images of 30 multiply lensed background galaxies, spanning a wide redshift range, 1.0 < z < 5.5. The resulting mass map is more circular in projection than the clumpy distribution of cluster galaxies, and the light is more concentrated than the mass within r < 50 kpc h -1 . The projected mass profile flattens steadily toward the center with a shallow mean slope of d log Σ/ d log r ≃ -0.55 ± 0.1, over the observed range r < 250 kpc h -1 , matching well an NFW profile, but with a relatively high concentration, C vir = 8.2 . A softened isothermal profile ( r core = 20 ± 2'') is not conclusively excluded, illustrating that lensing constrains only projected quantities. Regarding cosmology, we clearly detect the purely geometric increase of bend angles with redshift. The dependence on the cosmological parameters is weak owing to the proximity of A1689, z = 0.18, constraining the locus, Ω M + Ω Λ ≤ 1.2. This consistency with standard cosmology provides independent support for our model, because the redshift information is not required to derive an accurate mass map. Similarly, the relative fluxes of the multiple images are reproduced well by our best-fitting lens model.