Dissecting the gravitational lens B1608+656. I. Lens potential reconstruction

Strong gravitational lensing is a powerful technique for probing galaxy mass distributions and for measuring cosmological parameters. Lens systems with extended source-intensity distributions are particularly useful for this purpose since they provide additional constraints on the lens potential (mass distribution). We present a pixelated approach to modeling the lens potential and source-intensity distribution simultaneously. The method makes iterative and perturbative corrections to an initial potential model. For systems with sources of sufficient extent such that the separate lensed images are connected by intensity measurements, the accuracy in the reconstructed potential is solely limited by the quality of the data. We apply this potential reconstruction technique to deep Hubble Space Telescope observations of B1608+656, a four-image gravitational lens system formed by a pair of interacting lens galaxies. We present a comprehensive Bayesian analysis of the system that takes into account the extended source-intensity distribution, dust extinction, and the interacting lens galaxies. Our approach allows us to compare various models of the components of the lens system, which include the point-spread function (PSF), dust, lens galaxy light, source-intensity distribution, and lens potential. Using optimal combinations of the PSF, dust, and lens galaxy light models, we successfully reconstruct both the lens potential and the extended source-intensity distribution of B1608+656. The resulting reconstruction can be used as the basis of a measurement of the Hubble constant. As an illustration of the astrophysical applications of our method, we use our reconstruction of the gravitational potential to study the relative distribution of mass and light in the lensing galaxies. We find that the mass-to-light ratio for the primary lens galaxy is (2.0 ± 0.2)h M_® L ^-1 _B,®, within the Einstein radius (3.9 h^-1 kpc), in agreement with what is found for noninteracting lens galaxies at the same scales.