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

We have measured the kinematic profile of the early-type (E/S0) lens galaxy in the system 0047-281 (z=0.485) with the {\sl Echelle Spectrograph and Imager} (ESI) on the W.M. Keck--II Telescope, as part of the {\sl Lenses Structure and Dynamics (LSD) Survey}. The central velocity dispersion is σ=229\pm 15 \kms, and the dispersion profile is nearly flat to beyond one effective radius (R_e). Surface photometry of the lens galaxy is measured from {\sl Hubble Space Telescope} images. From the offset from the local Fundamental Plane (FP), we measure an evolution of the effective mass-to-light ratio of Δ\log M/L_B=-0.37\pm0.06 between z=0 and z=0.485, consistent with the observed evolution of field E/S0 galaxies. (We assume h_{65}=1,Ω_{\rm m}=0.3 and Ω_Λ=0.7 throughout.) Gravitational lens models provide a mass of M_{\rmE}=(4.06\pm0.20)\times 10^{11} h_{65}^{-1}M_\odot inside the Einstein radius of R_{\rm E}=(8.70\pm0.07) h_{65}^{-1} kpc. This allows us to break the degeneracy between velocity anisotropy and density profile, typical of dynamical models for E/S0 galaxies. We find that constant M/L model, even with strongly tangential anisotropy of the stellar velocity ellipsoid, are excluded at >99.9%CL. The total mass distribution inside R_{\rm E} can be described by a single power-law density profile, ρ_t\propto r^{-γ'}, with an effective slope γ'=1.90^{+0.05}_{-0.23} (68%CL; \pm0.1 systematic error). Two-component models yield an upper limit (68% CL) of γ\le 1.55(1.12) on the power-law slope of the dark-matter density profile and a projected dark-matter mass fraction of 0.41(0.54)^{+0.15}_{-0.05}(^{+0.09}_{-0.06}) (68% CL) inside R_{\rm E}, for Osipkov--Merritt models with anisotropy radius r_i=\infty(R_e).