The stellar velocity dispersion of the lens galaxy in MG2016+112 at z=1.004

Abstract

We present a direct measurement of the stellar velocity dispersion of the early-type lens galaxy D in the system MG2016+112 (z=1.004), determined from a spectrum 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 effective circular aperture with a radius of 0.65'', corresponding to a central velocity dispersion of sigma=328+-32 km/s. From a Hubble Space Telescope F160W-band image, we measure the effective radius and effective surface brightness in order to determine the offset of the lens galaxy with respect to the local Fundamental Plane. The offset corresponds to an evolution of the rest-frame effective mass-to-light ratio of Delta log(M/L_B)=-0.62+-0.08 from z=0 to z=1.004. By interpreting colors and offset of the FP with two independent stellar population synthesis models, we obtain a single-burst equivalent age of 2.8+-0.8 Gyr (i.e. z_f>1.9) and a supersolar metallicity of log[Z/Z_sun]=0.25+-0.25. The lens galaxy is therefore a massive elliptical dominated by an old and metal rich stellar population at z>1. The excellent agreement of the stellar velocity dispersion with that predicted from recent lens models confirms that the angular separation of the multiple images of the background 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 indicate that this system is a proto-cluster, in formation around galaxy D, responsible for the ~10% external shear inferred from the strong lens models.