2006ApJ...640..662T

We use HST images to derive effective radii and effective surface brightnesses of 15 early-type (E+S0) lens galaxies identified by the SLACS Survey. Our measurements are combined with stellar velocity dispersions from the SDSS database to investigate for the first time the distribution of lens galaxies in the fundamental plane (FP) space. Accounting for selection effects (top priority to the largest Einstein radii and thus approximately to the largest velocity dispersions, σ≳240 km/s) and for passive evolution, the distribution of the lens galaxies inside the FP is indistinguishable from that of the parent sample of SDSS galaxies. We conclude that SLACS lenses are a fair sample of high velocity dispersion E+S0s. By comparing the central stellar velocity dispersion (σ) with the velocity dispersion that best fits the lensing models (σ_SIE) we find <f_SIE>≡<σ/σ_SIE≥1.01±0.02 with 0.065 rms scatter. We infer that within the Einstein radii (typically R_e/2) the SLACS lenses are very well approximated by isothermal ellipsoids, requiring a fine tuning of the stellar and dark matter distribution (the bulge-halo ``conspiracy''). Interpreting the offset from the local FP in terms of evolution of the stellar mass-to-light ratio, we find dlog(M/L<sub>B</sub>)/dz=-0.69±0.08 (rms 0.11) consistent with the rate found for field E+S0s and with most of the stars being old (z<sub>f</sub>>2) and less than ∼10% of the stellar mass having formed below z=1. We discuss our results in the context of formation mechanisms such as collisionless (``dry'') mergers.