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 pro le 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 sigma = 229 +/- 15 km s(-1), and the dispersion pro le is nearly at to beyond one effective radius (R-e). No significant streaming motion is found. Surface photometry of the lens galaxy is measured from Hubble Space Telescope images. From the off-set from the local fundamental plane (FP), we measure an evolution of the effective mass-to-light ratio (M/L) of Delta log M/L-B = -0.37 +/- 0.06 between z = 0 and 0.485, consistent with the observed evolution of field E/S0 galaxies. (We assume h(65) = 1, Omega(m) = 0.3, and Omega(Lambda) = 0.7 throughout.) Gravitational lens models provide a mass of M-E = (4.06 +/- 0.20) x 10(11) h(65)(-1) M. inside the Einstein radius of R-E = (8.70 +/- 0.07) 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 models, even with strongly tangential anisotropy of the stellar velocity ellipsoid, are excluded at more than 99.9% CL. The total mass distribution inside R-E can be described by a single power-law density profile, rho(t) proportional to r-gamma', with an effective slope gamma' = 1.90(-0.23)(+0.05) (68% CL; +/-0.1 systematic error). Two-component models yield an upper limit (68% CL) of gamma less than or equal to 1.55(1.12) on the power-law slope of the dark matter density pro le and a projected dark matter mass fraction of 0.41(0.54)(-0.05)(+0.15)((+0.09)(-0.06)) (68% CL) inside R-E, for Osipkov-Merritt models with anisotropy radius r(i) = infinity (R-e). The stellar M-*/L values derived from the FP agree well with the maximum allowed value from the isotropic dynamical models (i.e., the maximum-bulge solution). The fact that both lens systems 0047-281 (z = 0.485) and MG 2016+112 (z = 1.004) are well described inside their Einstein radii by a constant-M-*/L stellar mass distribution embedded in a nearly logarithmic potential with an isotropic or a mildly radially anisotropic dispersion tensor could indicate that E/S0 galaxies underwent little structural evolution at z less than or similar to 1 and have a close-to-isothermal total mass distribution in their inner regions. Whether this conclusion can be generalized, however, requires the analysis of more systems. We briefly discuss our results in the context of E/S0 galaxy formation and cold dark matter simulations.