Strong‐Lensing Analysis of A1689 from Deep Advanced Camera Images

We analyse deep multi-colour Advanced Camera images of the largest knowngravitational lens, A1689. Radial and tangential arcs delineate the criticalcurves in unprecedented detail and many small counter-images are found near thecenter of mass. We construct a flexible light deflection field to predict theappearance and positions of counter-images. The model is refined as newcounter-images are identified and incorporated to improve the model, yielding atotal of 106 images of 30 multiply lensed background galaxies, spanning a wideredshift range, 1.0$<$z$<$5.5. The resulting mass map is more circular inprojection than the clumpy distribution of cluster galaxies and the light ismore concentrated than the mass within $r<50kpc/h$. The projected mass profileflattens steadily towards the center with a shallow mean slope of$d\log\Sigma/d\log r \simeq -0.55\pm0.1$, over the observed range,r$<250kpc/h$, matching well an NFW profile, but with a relatively highconcentration, $C_{vir}=8.2^{+2.1}_{-1.8}$. A softened isothermal profile($r_{core}=20\pm2$\arcs) is not conclusively excluded, illustrating thatlensing constrains only projected quantities. Regarding cosmology, we clearlydetect the purely geometric increase of bend-angles with redshift. Thedependence on the cosmological parameters is weak due to the proximity ofA1689, $z=0.18$, constraining the locus, $\Omega_M+\Omega_{\Lambda} \leq 1.2$.This consistency with standard cosmology provides independent support for ourmodel, because the redshift information is not required to derive an accuratemass map. Similarly, the relative fluxes of the multiple images are reproducedwell by our best fitting lens model.