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The Optical Properties of Gravitational Lens Galaxies as a Probe of Galaxy Structure and Evolution

We combine photometry and lens modeling to study the properties of 17 gravitational lens galaxies between z=0.1 and 1. Most of the lens galaxies are passively evolving early-type galaxies, with a few spirals. The colors, scale lengths, and ellipticities of lens galaxies are similar to those of the general population of early-type galaxies, although there may be a deficit of apparently round lens galaxies produced by the inclination dependence of lensing cross sections. The projected mass distributions are aligned with the projected light distributions to <~ 10 deg, except in the presence of a strong external tidal perturbation, suggesting that dark matter halos have orbits that are significantly modified by interactions with the baryonic component and are not far out of alignment with the stars. Lens galaxies obey image separation/lens luminosity correlations analogous to the Faber-Jackson and Tully-Fisher relations, which are consistent with standard dark matter lens models. The lens galaxy mass-to-light ratios decrease with redshift as d(log M/L_B)/dz = -0.3\pm0.1 (-0.5\pm0.1) for Omega_0=1 (0.1), thus providing direct evidence of passive evolution for a sample of early-type galaxies in low-density environments. The evolution-corrected mass-to-light ratios are generally larger than predicted by constant M/L dynamical models, although there is significant scatter; with improved photometry, lens galaxy mass-to-light ratios would better distinguish between constant M/L and dark matter models. These conclusions are limited primarily by the quality of lens galaxy photometry.