Detection of a Dark Substructure through Gravitational Imaging

We report the detection of a dark substructure ‐ undetected in the HST-ACS F814W image ‐ in the gravitational lens galaxy SDSSJ0946+1006 (the “Double Einstein Ring”), through direct gravitational imaging. The lens galaxy is of particular int erest because of its relative high inferred fraction of dark matter inside the effective radius. The detection is based on a Bayesian grid reconstruction of the two-dimensional surface density of the galaxy inside an annulus around its Einstein radius. The detection of a small mass concentration in the surface density maps has a strong statistical significance. We confirm this de tection by modeling the substructure with a tidally truncated pseudo-Jaffe density profile; in that case the substructure mass is Msub = (3.51±0.15)×10 9 M⊙, located at (−0.651±0.038, 1.040±0.034)”, precisely where also   