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L.V.E. Koopmans, T. Treu (Caltech)
\noindent The great beauty of galaxy dynamics and strong gravitational lensing stems from both being solely governed by the gravitational potential, the physics of which is well understood in the weak limit. The accuracy of galaxy mass models is therefore determined only by a lack of accurate observations, a poor understanding of the dark-matter distribution and degeneracies in the mass phase-space density models.
\noindent Because the stellar dynamics of gravitational lens galaxies and the properties of the lensed images both depend on the same gravitational potential, their combination results in a powerful tool that potentially can break some of these degeneracies and measure the properties of the baryonic and dark-matter distributions in galaxies out to z~1.
\noindent Recently, we have started a program to measure the stellar kinematics of a dozen gravitational lens galaxies between z=0.1--1.0, using ESI on Keck--II. I will present the first results from this program, showing the exquisite kinematic data that can be obtained from several of these lens galaxies. I will then shortly discuss three areas where this data can be used in combination with lensing constraints: (i) improve our understanding of galaxy evolution, (ii) reduce systematics errors in the value of the Hubble Constant from gravitational lens time-delays and (iii) improve constraints on gravitational lens statistics from large strong-lens surveys, such as the Cosmic Lens All-Sky Survey (CLASS) and the SDSS, as well as from the soon anticipated surveys with the ACS on HST.
\bigskip \noindent LVEK is supported by NSF~AST--9900866 and STScI~GO--06543.03--95A.