The Structure and Dynamics of Luminous and Dark Matter in the Early‐Type Lens Galaxy of 0047−281 atz= 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 { Echelle Spectrograph and Imager} (ESI) on the W.M. Keck--II Telescope, as part of the { Lenses Structure and Dynamics (LSD) Survey}. The central velocity dispersion is σ=229± 15 , and the dispersion profile is nearly flat to beyond one effective radius (R_e). Surface photometry of the lens galaxy is measured from { 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 Δlog M/L_B=-0.37±0.06 between z=0 and z=0.485, consistent with the observed evolution of field E/S0 galaxies. (We assume h₆₅=1,Ωₘ=0.3 and Ω_Λ=0.7 throughout.) Gravitational lens models provide a mass of M_{}=(4.06±0.20)× 10¹¹ h₆₅⁻¹M_ inside the Einstein radius of RE=(8.70±0.07) h₆₅⁻¹ 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 RE can be described by a single power-law density profile, ρ_t∝ r-γ', with an effective slope γ'=1.90+0.05-0.23 (68%CL; ±0.1 systematic error). Two-component models yield an upper limit (68% CL) of γ≤ 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 RE, for Osipkov--Merritt models with anisotropy radius r_i=∞(R_e).