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 <gamma'> = 2.01(-0.03)(+0.02) (68% CL; rho(tot) proportional to r(-gamma')) < 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 sigma(gamma') = 0.12. (2) The average position-angle difference between the light distribution and the total mass distribution is found to be <Delta theta > = 0 degrees +/- 3 degrees (rms of 10 degrees), setting an upper limit of <gamma(ext)> less than or similar to 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 sigma greater than or similar to 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 greater than or similar to 0.3, we find no significant evolution of the total density slope inside one effective radius for galaxies with sigma(ap) >= 200 km s(-1): a linear fit gives alpha(gamma') equivalent to d <gamma'>/ dz = 0.23 +/- 0.16 (1 sigma) 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.