Stellar and total mass in early-type lensing galaxies

For 18 well-observed gravitationally lensed QSOs, we compare new nonparametric mass profiles for the lensing galaxies with stellar population models derived from published Hubble Space Telescope photometry. The large volume of parameter space searched-with respect to the possible star formation histories-allows us to infer robust estimates and uncertainties for the stellar masses. The most interesting results are (1) the transition from little or no dark matter in the inner regions (≤r_e) to dark matter dominating on the ∼5r_e scale (∼20 kpc) is clearly seen in massive elliptical galaxies; (2) such a trend is not seen in lower mass galaxies, so that the stellar content dominates the mass budget out to ∼5r_e; and (3) the radial gradient in the dark matter fraction for these intermediate-redshift galaxies agrees with published data on nearby galaxies. This result can help reconcile the discrepancies found in recent estimates of dark matter in elliptical galaxies using different techniques (e.g., planetary nebulae vs. X-ray). The observed trend suggests the stellar component in massive galaxies extends farther out in terms of the dark matter scale radius.