The Sloan Lens ACS Survey. III. The Structure and Formation of Early-Type Galaxies and Their Evolution since z ~ 1

We present a joint gravitational lensing and stellar-dynamical analysis of 15 massive field early-type galaxies selected from the Sloan Lens ACS (SLACS) Survey. The following numerical results are found: (1) A joint likelihood gives an average logarithmic density slope for the total mass density of <γ^'>=2.01^+0.02_-0.03 (68% CL; ρ_tot~r^-γ') inside <R_Einst>=4.2+/-0.4 kpc (rms of 1.6 kpc) for isotropic models. The inferred intrinsic rms spread in logarithmic density slopes is σ_γ^'=0.12. (2) The average position-angle difference between the light distribution and the total mass distribution is found to be <Δθ>=0^deg+/-3^deg (rms of 10°), setting an upper limit of <γ_ext><~0.035 on the average external shear. The total mass has an average ellipticity <q_SIE>=0.78+/-0.03 (rms of 0.12), which correlates extremely well with the stellar ellipticity, q_*, resulting in <q_SIE/q_*>=0.99+/-0.03 (rms of 0.11) for σ>~225 km s^-1 omitting three S0 lens galaxies. (3) The average projected dark matter mass fraction is inferred to be <f_DM>=0.25+/-0.06 (rms of 0.22) inside <R_Einst>, using the stellar mass-to-light ratios derived from the fundamental plane as priors. (4) Combined with results from the Lenses Structure and Dynamics (LSD) Survey at z>~0.3, we find no significant evolution of the total density slope inside one effective radius for galaxies with σ_ap>=200 km s^-1: a linear fit gives α_γ^'≡d<γ^'>/dz=0.23+/-0.16 (1 σ) for the range z=0.08-1.01. The small scatter and absence of significant evolution in the inner density slopes suggest a collisional scenario in which gas and dark matter strongly couple during galaxy formation, leading to a total mass distribution that rapidly converges to dynamical isothermality.