The Stellar Velocity Dispersion of the Lens Galaxy in MG 2016+112 at [CLC][ITAL]z[/ITAL][/CLC] = 1.004

We present a direct measurement of the stellar velocity dispersion of theearly-type lens galaxy D in the system MG2016+112 (z=1.004), determined from aspectrum obtained with the Echelle Spectrograph and Imager (ESI) on the W.M.Keck-II Telescope, as part of the Lenses Structure and Dynamics (LSD) Survey.We find a velocity dispersion of sigma_ap=304+-27 km/s inside an effectivecircular aperture with a radius of 0.65'', corresponding to a central velocitydispersion of sigma=328+-32 km/s. From a Hubble Space Telescope F160W-bandimage, we measure the effective radius and effective surface brightness inorder to determine the offset of the lens galaxy with respect to the localFundamental Plane. The offset corresponds to an evolution of the rest-frameeffective mass-to-light ratio of Delta log(M/L_B)=-0.62+-0.08 from z=0 toz=1.004. By interpreting colors and offset of the FP with two independentstellar population synthesis models, we obtain a single-burst equivalent age of2.8+-0.8 Gyr (i.e. z_f>1.9) and a supersolar metallicity oflog[Z/Z_sun]=0.25+-0.25. The lens galaxy is therefore a massive ellipticaldominated by an old and metal rich stellar population at z>1. The excellentagreement of the stellar velocity dispersion with that predicted from recentlens models confirms that the angular separation of the multiple images of thebackground QSO is predominantly due to the lens galaxy, and not to a massive``dark cluster'', in agreement with recent weak lensing and X-ray observations.However, the significant overdensity of galaxies in the field might indicatethat this system is a proto-cluster, in formation around galaxy D, responsiblefor the ~10% external shear inferred from the strong lens models.Comment: Accepted for publication in ApJ Letters -- conclusions unchanged, revision of figure