The Internal Structure and Formation of Early-type Galaxies: the Gravitational–lens System Mg2016+112 at Z=1.004

We combine our recent measurements of the velocity dispersion and the surface brightness profile 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 that: i) dark matter accounts for >50% of the total mass within the Einstein radius (99% CL), whereas ∼75% is the more likely contribution. In particular, we can exclude at the 8–σ level that mass follows light inside the Einstein radius with a constant mass-to-light ratio (M/L). ii) the total mass distribution inside the Einstein radius is well-described by a density profile ∝ r ′ with an effective slope γ = 2.0± 0.1± 0.1, including random and systematic uncertainties. iii) 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 >60% (99% CL) on the fraction of dark matter within the Einstein radius. 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 γi < 1.4 (68% CL), only marginally consistent with the highest-resolution cold dark matter simulations that indicate γi∼1.5. This might indicate that either 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. We briefly discuss the importance of our results for lens statistics and the determination of the Hubble Constant from gravitational-lens time delays. Subject headings: gravitational lensing — galaxies: elliptical and lenticular, cD — galaxies: evolution —galaxies: formation — galaxies: structure