Gravitational lensing at milliarcsecond angular resolution

Gravitational lensing is a powerful tool for studying the mass content in distant galaxies, but also for performing a detailed study of high-redshift sources. In this thesis, we use milliarcsecond angular resolution observations of radio-loud gravitationally lensed sources to investigate both the properties of lensing galaxies and background sources. We present high-resolution global Very Long Baseline Interferometric (VLBI) observations of the gravitationally lensed radio source MG J0751+2716 (at z = 3.2), that shows evidence of both compact and extended structure (core-jet morphology) across several gravitational arcs. These data provide a wealth of observational constraints that are used to determine the inner (baryonic and dark matter) mass profile of a group of galaxies and also investigate the smoothness of the dark matter distribution on mas-scales. By complementing spectral line radio observations with optical and near-infrared imaging at high angular resolution, in the second chapter we investigate the cold molecular gas, stars and dust content in two high redshift galaxies (MG J0751+2716 and JVAS B1938+666). Moreover, by comparing two observations at milliarcsecond angular resolution separated by 15 years, we find evidence for proper motions observed for the first time in the gravitational lensing system MG B2016+112. The analysis of these data can constrain the formation model for super-massive black holes. Finally, we present a new pilot gravitational lens search in the VLBI survey mJIVE-20 in perspective of future surveys with the next generation of radio interferometers