The fundamental plane of gravitational lens galaxies and the evolution of early-type galaxies in low-density environments

Most gravitational lenses are early-type galaxies in relatively low density environments-a "field" rather than a "cluster" population. Their average p roperties are the mass-averaged properties of all early-type galaxies. We s how that held early-type galaxies with 0 < z < 1, as represented by the len s galaxies, lie on the same fundamental plane as those in rich clusters at similar redshifts. We then use the fundamental plane to measure the combine d evolutionary and K-corrections for early-type galaxies in the V, I, and H bands. Only for passively evolving stellar populations formed at z(f) grea ter than or similar to 2 (H-0 = 65 km s(-1) Mpc(-1), Omega (0), = 0.3, lamb da (0) = 0.7) can the lens galaxies be matched to the local fundamental pla ne. The high formation epoch and the lack of significant differences betwee n the held and cluster populations contradict many current models of the fo rmation history of early-type galaxies. Lens galaxy colors and the fundamen tal plane provide good photometric redshift estimates with an empirical acc uracy of [z(FP) - z(l)]= -0.04 +/- 0.09 for the 20 lenses with known redshi fts. A mass model dominated by dark matter is more consistent with the data than either an isotropic or radially anisotropic constant MIL mass model, and a radially anisotropic model is better than an isotropic model.