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We use a sample of 53 massive early-type strong gravitational lens galaxieswith well-measured redshifts (ranging from z=0.06 to 0.36) and stellar velocitydispersions (between 175 and 400 km/s) from the Sloan Lens ACS (SLACS) Surveyto derive numerous empirical scaling relations. The ratio between centralstellar velocity dispersion and isothermal lens-model velocity dispersion isnearly unity within errors. The SLACS lenses define a fundamental plane (FP)that is consistent with the FP of the general population of early-typegalaxies. We measure the relationship between strong-lensing mass M_lens withinone-half effective radius (R_e/2) and the dimensional mass variable M_dim =G^-1 sigma_e2^2 R_e/2 to be log_10 [M_lens/10^11 M_Sun] = (1.03 +/- 0.04)log_10 [M_dim/10^11 M_Sun] + (0.54 +/- 0.02) (where sigma_e2 is the projectedstellar velocity dispersion within R_e/2). The near-unity slope indicates thatthe mass-dynamical structure of massive elliptical galaxies is independent ofmass, and that the "tilt" of the SLACS FP is due entirely to variation in total(luminous plus dark) mass-to-light ratio with mass. Our results imply thatdynamical masses serve as a good proxies for true masses in massive ellipticalgalaxies. Regarding the SLACS lenses as a homologous population, we find thatthe average enclosed 2D mass profile goes as log_10 [M(<R)/M_dim] = (1.10 +/-0.09) log_10 [R/R_e] + (0.85 +/- 0.03), consistent with an isothermal (flatrotation curve) model when de-projected into 3D. This measurement isinconsistent with the slope of the average projected aperture luminosityprofile at a confidence level greater than 99.9%, implying a minimumdark-matter fraction of f_DM = 0.38 +/- 0.07 within one effective radius.(abridged)

The Sloan Lens ACS Survey. VII. Elliptical Galaxy Scaling Laws from Direct Observational Mass Measurements