THE STRUCTURE AND DYNAMICS OF MASSIVE EARLY-TYPE GALAXIES: ON HOMOLOGY, ISOTHERMALITY, AND ISOTROPY INSIDE ONE EFFECTIVE RADIUS

Based on 58 SLACS strong-lens early-type galaxies (ETGs) with direct total-mass and stellar-velocity dispersion measurements, we find that inside one effective radius maassive elliptical galaxies with M-eff greater than or similar to 3 x 10(10) M-circle dot. are well approximated by a power-law ellipsoid, with an average logarithmic density slope of equivalent to -d log(rho(tot))/d log(r) = 2.085(-0.018)(+0.025) and sigma(gamma') less than or similar to 0.20(-0.02)(+0.04) (random error on mean) for isotropic orbits with beta(r) = 0, +/- 0.1 (syst.) intrinsic scatter (all errors indicate the 68% CL). We find no correlation of gamma'(LD) with galaxy mass (M-eff), rescaled radius (i.e., R-einst/R-eff) or redshift, despite intrinsic differences in density-slope between galaxies. Based on scaling relations, the average logarithmic density slope can be derived in an alternative manner, fully independent from dynamics, yielding = 1.959 +/- 0.077. Agreement between the two values is reached for = 0.45 +/- 0.25, consistent with mild radial anisotropy. This agreement supports the robustness of our results, despite the increase in mass-to-light ratio with total galaxy mass: M-eff proportional to L-V,eff(1.363 +/- 0.056). We conclude that massive ETGs are structurally close to homologous with close to isothermal total density profiles (less than or similar to 10% intrinsic scatter) and have at most some mild radial anisotropy. Our results provide new observational limits on galaxy formation and evolution scenarios, covering 4 Gyr look-back time.