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The SL2S Galaxy-scale Lens Sample. II. Cosmic evolution of dark and luminous mass in early-type galaxies

We present a joint gravitational lensing and stellar-dynamical analysis of 11 early-type galaxies (median deflector redshift $\zd=0.5$) from Strong Lenses in the Legacy Survey (SL2S). Using newly measured redshifts and stellar velocity dispersions from Keck spectroscopy with lens models from Paper I, we derive the total mass density slope inside the Einstein radius for each of the 11 lenses. The average total density slope is found to be $<γ'>= 2.16^{+0.09}_{-0.09}$ ($ρ_{\rm tot}\propto r^{-γ'}$), with an intrinsic scatter of $0.25^{+0.10}_{-0.07}$. We also determine the dark matter fraction for each lens within half the effective radius, and find the average projected dark matter mass fraction to be $0.42^{+0.08}_{-0.08}$ with a scatter of $0.20^{+0.09}_{-0.07}$ for a Salpeter IMF. By combining the SL2S results with those from the Sloan Lens ACS Survey (median $\zd=0.2$) and the Lenses Structure and Dynamics survey (median $\zd=0.8$), we investigate cosmic evolution of $γ'$ and find a mild trend $\partial<γ'>/\partial\zd = -0.25^{+0.10}_{-0.12}$. This suggests that the total density profile of massive galaxies has become slightly steeper over cosmic time. If this result is confirmed by larger samples, it would indicate that dissipative processes played some role in the growth of massive galaxies since $z\sim1$.