Investigating the structure of cluster galaxies with combined strong lensing and stellar kinematics.

Strong lensing (SL) is a powerful probe of the dark matter mass distribution in the cores of galaxy clusters, providing us with insights into its nature, and with tests of CDM cosmology. SL models predict an excess of galaxy-galaxy strong lensing (GGSL) events in clusters compared with cosmological simulations: this is reflected by a higher compactness for the observed cluster galaxies with respect to simulations. The mismatch could point towards new physics beyond CDM. In this talk, I will discuss how the determination of the compactness of the cluster members in SL models is hindered by parametric degeneracies, which are broken by studying GGSL events. I will present new SL models for several galaxy lenses within the clusters MACSJ0416 and MACSJ1206. We extend current studies on lens galaxies to lower mass limits and confirm the high compactness predicted for cluster galaxies by recent SL works. Secondly, I will show how enhancing SL models with stellar kinematics allows us to probe the structure of the cluster galaxies. I will present new measurements of the line-of-sight velocity dispersion for several hundreds of members, from deep VLT/MUSE observations of massive clusters.