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

Abstract

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^-1, 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 Hubble Space Telescope images. From the offset from the local fundamental plane (FP), we measure an evolution of the effective mass-to-light ratio (M/L) of Δlog(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 h₆₅=1, Ω_m=0.3, and Ω_Λ=0.7 throughout.) Gravitational lens models provide a mass of M_E=(4.06+/-0.20)×10¹¹h^{- 1}₆₅ M_solar inside the Einstein radius of R_E=(8.70+/-0.07)h^-1₆₅ 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, 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 R_E, for Osipkov-Merritt models with anisotropy radius r_i=∞(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<~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.

Based on observations collected at W. M. Keck Observatory, which is operated jointly by the California Institute of Technology and the University of California, and with the NASA/ESA Hubble Space Telescope, obtained at STScI, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under NASA contract NAS 5-26555.