The internal structure and formation of early-type galaxies:: The gravitational lens system MG 2016+112 at z = 1.004

We combine our recent measurements of the velocity dispersion and the surface brightness pro le of the lens galaxy D in the system MG2016+112 (z = 1.004) with constraints from gravitational lensing to study its internal mass distribution. We find the following: (1) dark matter accounts for more than 50% of the total mass within the Einstein radius (99% confidence limit [CL]), whereas similar to75% is the more likely contribution. In particular, we can exclude at the 8 sigma level that mass follows light inside the Einstein radius with a constant mass-to-light ratio (M/L). (2) The total mass distribution inside the Einstein radius is well described by a density profile proportional to r(-gamma)', with an effective slope gamma' = 2.0 +/- 0.1 +/- 0.1, including random and systematic uncertainties. (3) The offset of galaxy D from the local fundamental plane independently constrains the stellar M/L and matches the range derived from our models, leading to a more stringent lower limit of more than 60% on the fraction of dark matter within the Einstein radius (99% CL). Under the assumption of adiabatic contraction, we show that the inner slope of the dark matter halo before the baryons collapsed to form the lens galaxy is gamma(i) < 1.4 (68% CL), only marginally consistent with the highest resolution cold dark matter simulations that indicate gamma(i) similar to 1.5. This might indicate either that adiabatic contraction is a poor description of early-type galaxy formation or that additional processes play a role as well. Indeed, the apparently isothermal density distribution inside the Einstein radius is not a natural outcome of adiabatic contraction models, where it appears to be a mere coincidence. By contrast, we argue that isothermality might be the result of a stronger coupling between luminous and dark matter, possibly the result of (incomplete) violent relaxation processes during the formation of the innermost regions of the galaxy. Hence, we conclude that galaxy D appears already relaxed similar to 8 Gyr ago. We briefly discuss the importance of our results for lens statistics and the determination of the Hubble constant from gravitational lens time delays.