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

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  • 2 January 2002
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 ESI on the Keck, as part of the Lenses Structure and Dynamics (LSD) Survey. We find a 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 HST F160W image, we measure the effective radius and effective surface brightness in order to determine the offset of galaxy D 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 evolved stellar population (1.1x10^12 M_sun within the Einstein radius and an integrated luminosity of (1.6+-0.2)x10^11 L_B,sun). The excellent agreement of the stellar velocity dispersion with that inferred from recent lens models confirms that the angular separation of the multiple images of the background QSO is predominantly due to galaxy D, and not due 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 the old massive elliptical galaxy D and responsible for the \~10 percent external shear inferred from the strong lens models.

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