A More Fundamental Plane

We present a comparative scaling analysis of 37 early-type gravitational-lens galaxies from the Sloan Lens ACS Survey, based on Hubble Space Telescope imaging and Sloan Digital Sky Survey spectroscopy. By measuring masses via strong lensing and velocity dispersions from stellar kinematics, we construct both the fundamental plane (FP) scaling relation, and an analogous "mass plane" (MP) that replaces surface brightness with mass density. We show that the residuals about the best-fit FP are significantly correlated with the lensing-determined total mass-to-light ratio, giving a clear observational explanation for the intrinsic thickness of the FP. We find that in going from the FP to the MP, the plane's "tilt" -- i.e. its deviation from the simplest virial expectation -- is significantly reduced, and its tightness is increased. This result implies that galaxies form a substantially homologous population in their mass structure, and that the tilt of the FP is due primarily to a systematically varying mass-to-light ratio rather than to structural non-homology. The intrinsic tightness of the MP suggests the use of this plane in lens galaxies as a cosmological distance indicator independent of luminosity-evolution effects.