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

Strong lensing (SL) is a powerful probe of the dark matter (DM) mass distribution in the cores of galaxy clusters, providing us with stringent tests of the cold DM (CDM) paradigm. SL models predict an excess of galaxy-galaxy SL events for observed galaxy clusters compared to simulated data based on cosmological simulations: this is reflected by a higher compactness for the observed cluster galaxies with respect to their simulated counterparts. We address this discrepancy by building improved SL models of cluster galaxies. We describe a SL model of the massive cluster Abell S1063, where the properties of the cluster galaxies are described more accurately than in previous studies using a parametrisation based on the Fundamental Plane relation, which we calibrated based on the observed kinematic properties of the members. We present new SL models for three galaxy lenses within the clusters MACS J0416.1-2403 and MACS J1206.2-0847; we have measured their truncation radius and their stellar-to-total mass fraction, extending current studies on lens galaxies to lower mass limits. We compare our results with a suite of cosmological hydrodynamical simulations, testing the effects of the resolution and of the feedback set-up, and confirming the lower compactness predicted for simulated cluster galaxies. This persistent mismatch could point towards new physics beyond CDM.