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 h 0 i 2:01 0:02 0:03 (68% CL; tot / r 0 ) inside hR Einst i 4:2 AE 0:4 kpc (rms of 1.6 kpc) for isotropic models. The inferred intrinsic rms spread in logarithmic density slopes is 0 0:12. (2) The average position-angle difference between the light distribution and the total mass distribution is found to be hi 0 AE 3 (rms of 10 ), setting an upper limit of h ext i P 0:035 on the average external shear. The total mass has an average ellipticity hq SIE i 0:78 AE 0:03 (rms of 0.12), which correlates extremely well with the stellar ellipticity, q , resulting in hq SIE /q i 0:99 AE 0:03 (rms of 0.11) for k 225 km s 1 omitting three S0 lens galaxies. (3) The average projected dark matter mass fraction is inferred to be h f DM i 0:25 AE 0:06 (rms of 0.22) inside hR Einst i, 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 k 0:3, we find no significant evolution of the total density slope inside one effective radius for galaxies with ap ! 200 km s 1 : a linear fit gives 0 dh 0 i/dz 0:23 AE 0:16 (1 ) for the range z 0:08Y1.01. 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.