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 $<\gamma'>=
2.16^{+0.09}_{-0.09}$ ($\rho_{\rm tot}\propto r^{-\gamma'}$), 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 $\gamma'$ and find a mild trend $\partial<\gamma'>/\partial\zd =
-0.25^{+0.10}_{-0.12}$. 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 $z\sim1$.
