The SL2S Galaxy-scale Gravitational Lens Sample. I. The alignment of
  mass and light in massive early-type galaxies at z=0.2-0.9
  We study the relative alignment of mass and light in a sample of 16 massive
early-type galaxies at z=0.2-0.9 that act as strong gravitational lenses. The
sample was identified from deep multi-band images obtained as part of the
Canada France Hawaii Telescope Legacy Survey as part of the Strong Lensing
Legacy Survey (SL2S). Higher resolution follow-up imaging is available for a
subset of 10 systems. We construct gravitational lens models and infer total
enclosed mass, elongation, and position angle of the mass distribution. By
comparison with the observed distribution of light we infer that there is a
substantial amount of external shear with mean value $<\gamma_{\rm ext}>= 0.12
\pm 0.05$, arising most likely from the environment of the SL2S lenses. In a
companion paper, we combine these measurements with follow-up Keck spectroscopy
to study the evolution of the stellar and dark matter content of early-type
galaxies as a function of cosmic time.
