The Internal Structure and Formation of Early‐Type Galaxies: The Gravitational Lens System MG 2016+112 atz= 1.004

We combine our recent measurements of the velocity dispersion and the surface brightness profile of the lens galaxy D in the system MG 2016+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 ~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). (2) The total mass distribution inside the Einstein radius is well described by a density profile ~r-γ', with an effective slope γ'=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 γi