Two-dimensional kinematics of SLACS lenses - I. Phase-space analysis of the early-type galaxy SDSSJ2321-097 at z ~ 0.1

Abstract

We present the first results of a combined VLT VIMOS integral-field unit and Hubble Space Telescope (HST)/ACS study of the early-type lens galaxy SDSSJ2321-097 at z = 0.0819, extending kinematic studies to a look-back time of 1Gyr. This system, discovered in the Sloan Lens ACS Survey, has been observed as part of a VLT Large Programme with the goal of obtaining two-dimensional stellar kinematics of 17 early-type galaxies to z ~ 0.35 and Keck spectroscopy of an additional dozen lens systems. Bayesian modelling of both the surface brightness distribution of the lensed source and the two-dimensional measurements of velocity and velocity dispersion has allowed us, under the only assumptions of axisymmetry and a two-integral stellar distribution function (DF) for the lens galaxy, to dissect this galaxy in three dimensions and break the classical mass-anisotropy, mass-sheet and inclination-oblateness degeneracies. Our main results are that the galaxy (i) has a total density profile well described by a single power law with γ' = 2.06^+0.03_-0.06 (ii) is a very slow rotator (specific stellar angular momentum parameter λ_R = 0.075); (iii) shows only mild anisotropy (δ ~ 0.15); and (iv) has a dark matter contribution of ~30 per cent inside the effective radius. Our first results from this large combined imaging and spectroscopic effort with the VLT, Keck and HST show that the structure of massive early-type galaxies beyond the local Universe can now be studied in great detail using the combination of stellar kinematics and gravitational lensing. Extending these studies to look-back times where evolutionary effects become measurable holds great promise for the understanding of formation and evolution of early-type galaxies.

Based on observations made with ESO telescopes at the La Silla or Paranal Observatories under programme IDs 075.B-0226 and 177.B-0682 and on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS 5-26555.

E-mail: czoske@astro.rug.nl