The non-evolving internal structure of early-type galaxies: the case study SDSS J0728+3835 at z=0.206

We study the internal dynamical structure of the early-type lens galaxy SDSS J0728+3835 at z = 0.206. The analysis is based on two-dimensional kinematic maps extending out to 1.7 effective radii obtained from Keck spectroscopy, on lensing geometry and on stellar mass estimates obtained from multiband Hubble Space Telescope imaging. The data are modelled under the assumptions of axial symmetry supported by a two-integral distribution function, by applying the combined gravitational lensing and stellar dynamics code cauldron and by yielding high-quality constraints for an early-type galaxy at cosmological redshifts. Modelling the total density profile as a power law of the form , we find that it is nearly isothermal (logarithmic slope gamma' = 2.08+0.04(-0.02)) and quite flattened (axial ratio q = 0.60+0.08(-0.03)). The galaxy is mildly anisotropic (delta = 0.08 +/- 0.02) and shows a fair amount of rotational support, in particular towards the outer regions. We determine a dark matter fraction lower limit of 28 per cent within the effective radius. The stellar contribution to the total mass distribution is close to maximal for a Chabrier initial mass function (IMF), whereas for a Salpeter IMF the stellar mass exceeds the total mass within the galaxy inner regions. We find that the combination of a Navarro, Frenk and White dark matter halo with the maximally rescaled luminous profile provides a remarkably good fit to the total mass distribution over a broad radial range. Our results confirm and expand the findings of the Sloan Lens ACS Survey for early-type galaxies of comparable velocity dispersion (Sigma(SDSS) = 214 +/- 11 km s-1). The internal structure of SDSS J0728 is consistent with that of local early-type galaxies of comparable velocity dispersion as measured by the Spectrographic Areal Unit for Research on Optical Nebulae (SAURON) project, suggesting lack of evolution in the past two billion years.