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 \sim 75 % 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 ) .
( ii ) the total mass distribution inside the Einstein radius is well-described by a density profile \propto r ^ { - \gamma ^ { \prime } } with an effective slope \gamma ^ { \prime } = 2.0 \pm 0.1 \pm 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 % 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 } \sim 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 .
Hence , we conclude that galaxy D appears already relaxed \sim 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 .