The evolution of early-type galaxies: a strong lensing perspective

Early-type galaxies are believed to grow as a result of mergers, but the details of this process are still largely unknown. Do the mergers involve galaxies of comparable mass (major) or are they dominated by small systems (minor)? Is there dissipation (wet) or not (dry)? Different processes leave different signatures on the mass structure of early-type galaxies. Gravitational lensing provides a unique way to detect these signatures. The SL2S project measured the evolution of the mass profile of massive early-type galaxies during the last 7 billion years, including constraints on the mean density slope, dark matter fraction, inner dark matter slope and stellar IMF. Based on collected data, we find that theoretical models for the evolution of early-type galaxies through dry mergers alone are unable to reproduce the observed trends. Additional physical processes, likely related to baryonic physics, are necessary to match the entire set of observables.