The Structure and Dynamics of Luminous and Dark Matter in the Early-Type Lens Galaxy of 0047-281 at z=0.485

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Abstract
(Abridged) We have measured the kinematic profile of the early-type (E/S0) lens galaxy in the system 0047-281 (z=0.485) with ESI on Keck, as part of the Lenses Structure and Dynamics (LSD) Survey. The central velocity dispersion is sigma=229+-15 km/s, and the dispersion profile is nearly flat to beyond one effective radius (R_e). No significant streaming motion is found. Surface photometry of the lens galaxy is measured from HST images. From the offset from the local Fundamental Plane (FP), we measure an evolution of the effective mass-to-light ratio of \Delta\log M/L_B=-0.37+-0.06 between z=0 and z=0.485, consistent with the observed evolution of field E/S0 galaxies. Gravitational lens models provide a very accurate mass measurement inside the Einstein radius. This allows us to break the degeneracy between velocity anisotropy and density profile, typical of dynamical models for E/S0 galaxies. We find that both constant M/L and tangentially anisotropic dispersion models are excluded at >99.9 percent CL. The total mass distribution inside R_E can be described by a single power-law density profile, \rho_t r^-\gamma', with an effective slope \gamma'=1.90(+0.07/-0.20) (68% CL; +-0.1 systematic error). Two-component models yield an upper limit (68% CL) of \gamma<= 1.0(1.4) on the power-law slope of the dark-matter density profile and a projected dark-matter mass fraction of 0.52(0.42+-0.05) inside R_E, for Osipkov-Merritt models with anisotropy radius r_i=R_e(infinity). The stellar M_*/L values derived from the FP and dynamical models are found to agree very well.

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