Two-dimensional kinematics of SLACS lenses -: I.: Phase-space analysis of the early-type galaxy SDSS J2321-097 at z ≈ 0.1

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 SDSS J2321-097 at z = 0.0819, extending kinematic studies to a look-back time of 1 Gyr. 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 approximate to 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 p alpha r(-gamma') with gamma' = 2.06(-0.06)(+0.03); (ii) is a very slow rotator (specific stellar angular momentum parameter lambda(R) = 0.075); (iii) shows only mild anisotropy (delta approximate to 0.15); and (iv) has a dark matter contribution of similar to 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.