Mass makeup of galaxies

Lensing galaxies, unlike other samples of galaxies, are selected based on their masses and mass profile, rather than their colors or surface brightnesses. Therefore, gravitational lensing allows us to select galaxies based on their mass rather than their light, even at high redshift, without the need for medium resolution spectroscopy. Unfortunately, lensing by galaxy mass objects produces lensed sources with image separation of the order of 1” and a large fraction of the lensed sources by galaxies are unresolved using typical seeing-limited ground based observations. In this white paper we outline the need for a homogeneous lensed quasar study and we argue that this can be achieved using a new observational paradigm namely the use of relatively low-cost Adaptive Optics (AO) systems mounted on 2m class telescopes. We note that such instruments will have impact also ob galaxy-galaxy lensing studies.