The Sloan Lens ACS Survey. VI. Discovery and Analysis of a Double Einstein Ring1

We report the discovery of two concentric Einstein rings around the gravitational lens SDSS J0946+ 1006. The main lens is at redshift z(l) = 0.222, while the inner ring ( 1) is at redshift z(s1) 0.609 (R-Ein1 = 1.43 '' +/- 0.01 ''). The wider image separation ( R-Ein2 = 2.07 '' +/- 0.02 '') of the outer ring (2) implies a higher redshift than that of ring 1; the detection of ring 2 in the F814WACS filter implies an upper limit of z(s2) less than or similar to 6.9. The main lens can be described by a power- law total mass density profile rho(tot) proportional to r(-gamma ') with gamma ' = 2.00 +/- 0.03 and velocity dispersion sigma(SIE) = 287 +/- 5 km s(-1) ( the stellar velocity dispersion is sigma(v,*) = 284 +/- 24 km s(-1)). The strong lensing configuration is inconsistent with light traces mass. Adopting a prior on the stellar mass- to- light ratio from previous SLACS work, we infer a 73% +/- 9% dark matter fraction within the cylinder of radius equal to the effective radius of the lens. We find that, for the case of SDSS J0946+ 1006, the geometry of the two rings does not place interesting constraints on cosmography because of the suboptimal redshifts of lens and sources. We then consider the perturbing effect of the mass associated with ring 1 building a compound lens model. This introduces minor changes to the mass of the main lens and provides an estimate of z(s2) = 3.1(-1.0)(+2.0) and of the mass of the source responsible for ring 1 (sigma(SIE,s1) = 94(-47)(+27) km s(-1)). We conclude by examining the prospects of doing cosmography with a sample of 50 double rings, expected from future space- based surveys. Accounting for uncertainties in the mass profile of the lens and the effects of the perturber, we find that such a sample would constrain Omega(m) and within 10%, assuming flatness.