A more fundamental plane

We combine strong-lensing masses with SDSS stellar velocity dispersions and HST ACS effective (half-light) radii for 36 lens galaxies from the Sloan Lens ACS (SLACS) Survey to study the mass dependence of mass-dynamical structure in early-type galaxies. We find that over a 180-390 km s(-1) range in velocity dispersion, structure is independent of lensing mass to within 5%. This result suggests a systematic variation in the total (i.e., luminous plus dark matter) mass-to-light ratio as the origin of the tilt of the fundamental plane ( FP) scaling relationship between galaxy size, velocity dispersion, and surface brightness. We construct the FP of the lens sample, which we find to be consistent with the FP of the parent SDSS early-type galaxy population. We also present the first observational correlation between the mass-to-light ratio and residuals about the FP. Finally, we reformulate the FP in terms of surface mass density rather than surface brightness. By removing the complexities of stellar-population effects, this mass-plane formulation will facilitate comparison to numerical simulations and possible use as a cosmological distance indicator.