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 (68% CL; ρ tot ∝ r ) 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 = 0.12. (2) The average position-angle difference between the light distribution and the total mass distribution is found to be ⟨Δθ⟩ = 0 ° ± 3 ° (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 ≡ 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 .