The Sloan Lens Acs Survey. Vi: Discovery and Analysis of a Double Einstein Ring

We report the discovery of two concentric partial Einstein rings around the gravitational lens SDSSJ0946+1006, as part of the Sloan Lens ACS Survey. The main lens is at redshift zl = 0.222, while the inner ring (1) is at redshift zs1 = 0.609 and Einstein radius REin1 = 1.43 ± 0.01. The wider image separation (REin2 = 2.07±0.02′′) of the outer ring (2) implies that it is at higher redshift than Ring 1. Although no spectroscopic feature was detected in ∼ 9 hours of spectroscopy at the Keck I Telescope, the detection of Ring 2 in the F814W ACS filter implies an upper limit on the redshift of zs2 . 6.9. The lens configuration can be well described by a power law total mass density profile for the main lens ρtot ∝ r ′ with logarithmic slope γ = 2.00± 0.03 (i.e. close to isothermal), velocity dispersion σSIE = 287 ± 5 km s (in good agreement with the stellar velocity dispersion σv,∗ = 284± 24 km s) with little dependence upon cosmological parameters or the redshift of Ring 2. Using strong lensing constraints only we show that the enclosed mass to light ratio increases as a function of radius, inconsistent with mass following light. Adopting a prior on the stellar mass to light ratio from previous SLACS work we infer that 73± 9% of the mass is in form of dark matter within the cylinder of radius equal to the effective radius of the lens. We consider whether the double source plane configuration can be used to constrain cosmological parameters exploiting the ratios of angular distance ratios entering the set of lens equations. We find that constraints for SDSSJ0946+1006 are uninteresting due to the sub-optimal lens and source redshifts for this application. We then consider the perturbing effect of the mass associated with Ring 1 (modeled as a singular isothermal sphere) building a double lens plane compound lens model. This introduces minor changes to the mass of the main lens, allows to estimate the redshift of the Ring 2 (zs2 = 3.1 +2.0 −1.0), and the mass of the source responsible for Ring 1 (σSIE,s1 = 94 +27 −47 km s ). We conclude by examining the prospects of doing cosmography with a sample of 50 double source plane gravitational lenses, expected from future space based surveys such as DUNE or JDEM. Taking full account of the uncertainty in the mass density profile of the main lens, and of the effect of the perturber, and assuming known redshifts for both sources, we find that such a sample could be used to measure Ωm and w with 10% accuracy, assuming a flat cosmological model. Subject headings: Gravitational lensing – galaxies : Ellipticals and lenticulars, cD – galaxies: structure – galaxies: halos – cosmology: dark matter – cosmology: cosmological parameters