Mass Follows Light

We use comparisons between the shapes of gravitational lens galaxies and models for their mass distributions to derive statistical constraints on the alignment of the mass distribution relative to the observed lens galaxy and on the strength of tidal shear perturbations. The mass distributions are aligned with the luminous galaxies, with a ∆θ 2 1/2 < 10 • upper limit on the dispersion in the angle between the major axes. Statistical constraints, such as our bound on the misalignment between mass and light, are an important new approach to reducing the uncertainties in individual lens models, particularly for lenses used to estimate the Hubble constant.