Two-dimensional kinematics of SLACS lenses - I. Phase-space analysis of the early-type galaxy SDSS J2321−097 atz≈ 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≈ 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 λR= 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.