The Sloan Lens ACS Survey. IV. The Mass Density Profile of Early-Type Galaxies out to 100 Effective Radii

We present a weak-lensing analysis of 22 early-type (strong) lens galaxies, based on deep HST images obtained as part of the Sloan Lens ACS Survey. Using advanced techniques to control systematic uncertainties, we show that weak-lensing signal is detected out to ~300 h^(-1) kpc (at the mean lens redshift z = 0.2). We analyze blank control fields from COSMOS in the same manner, inferring that the residual systematic uncertainty in the tangential shear is less than 0.3%. A joint strong- and weak-lensing analysis shows that the average total mass density profile is consistent with isothermal (i.e., ρ ∝ r^(-2)) over two decades in radius (3-300 h^(-1) kpc, approximately 1-100 effective radii). This finding extends by over an order of magnitude in radius previous results, based on strong lensing and/or stellar dynamics, that luminous and dark components "conspire" to form an isothermal mass distribution. In order to disentangle the contributions of luminous and dark matter, we fit a two-component mass model (de Vaucouleurs+NFW) to the weak- and strong-lensing constraints. It provides a good fit to the data with only two free parameters: (1) the average stellar mass-to-light ratio M_*/L_V = 4.48 ± 0.46 h M_☉ L_☉^(-1) (at z = 0.2), in agreement with that expected for an old stellar population; (2) the average virial mass-to-light ratio M_(vir)/L_V = 246^(+101)_(-87) h M_☉ L_☉^(-1). Taking into account the scatter in the mass-luminosity relation, the latter result is in good agreement with semianalytical models of massive galaxy formation. The dark matter fraction inside the sphere of radius, the effective radius, is found to be 27% ± 4%. Our results are consistent with galaxy-galaxy lensing studies of early-type galaxies that are not strong lenses, in the 30-300 h^(-1) kpc radius range. Thus, within the uncertainties, our results are representative of early-type galaxies in general