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 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 km s, and the dispersion profile is nearly flat to beyond one effective radius (Re). No significant streaming motion is found. 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 massto-light ratio of ∆ logM/LB = −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 ΩΛ=0.7 throughout.) Gravitational lens models provide a very accurate mass measurement inside the Einstein radius of RE = 8.42 ± 0.07 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 both constant M/L and tangentially anisotropic dispersion models are excluded at >99.9% CL. The total mass distribution inside RE can be described by a single power-law density profile, ρt ∝ r −γ′ , with an effective slope γ = 1.90 −0.20 (68% CL; ±0.1 systematic error). Two-component models yield an upper limit (68% CL) of γ ≤ 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 RE, for Osipkov-Merritt models with anisotropy radius ri = Re(∞). The stellar M∗/L values derived from the FP and dynamical models are found to agree very well. The results of the LSD survey so far (0047−281 and MG2016+112 at z=1.004) indicate that E/S0 galaxies are well described inside their Einstein radii by a constant M∗/L stellar mass distribution embedded in a logarithmic potential, with an isotropic or a mildly radially anisotropic dispersion tensor. These results could indicate that E/S0 galaxies underwent little structural evolution at z . 1 and have a close-to-isothermal total mass distribution in their inner regions. We briefly discuss these results in the context of E/S0 galaxies formation and cold-dark-matter simulations. Subject headings: gravitational lensing — galaxies: elliptical and lenticular, cD — galaxies: evolution —galaxies: formation — galaxies: structure