Dynamical analysis of strong-lensing galaxy groups at intermediate redshift

We present VLT spectroscopic observations of seven newly discovered galaxy groups between 0.3 < z < 0.7. The groups were selected from the Strong Lensing Legacy Survey (SL2S), a survey that consists of a systematic search for strong lensing systems in the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS). We give details about the target selection, spectroscopic observations, and data reduction for the first release of confirmed SL2S groups. The dynamical analysis of the systems reveals that they are gravitationally bound structures, with at least 4 confirmed members and velocity dispersions between 300 and 800 km s(-1). Their virial masses are between 10(13) and 10(14) M-circle dot, so they can be classified as groups or low mass clusters. Most of the systems are isolated groups, except two of them that show evidence of an ongoing merger of two substructures. We find that weak lensing estimates of the group velocity dispersions are 50% greater than estimates based upon the radial velocities of its members, and conclude that the dynamics of baryonic matter is a good tracer of the total mass content in galaxy groups.