Studying galaxy evolution through strong gravitational lensing

The path of photons emitted by a distant source is perturbed when passing in proximity of a massive structure (a “lens”, or “deflector”) like a galaxy, a group, or a cluster of galaxies. This effect, called “strong gravitational lensing” makes the source appear brighter, displaced, distorted and, sometimes, multiply imaged. Nowadays, strong gravitational lensing represents a powerful probe in extragalactic astrophysics, thanks to two main properties. Firstly, the deflection of light rays only depends on the total mass distribution of the deflector (i.e., on its gravitational potential), and on the mutual angular-diameter distances between the source, the deflector, and the observer. Thus, the strong lensing observables - like the positions, the flux ratios, and the time delays between the multiple images - can be exploited to study the total, dark matter, and baryonic mass profiles of the deflectors and the geometry and content of the Universe. Secondly, the magnification effect can significantly boost the flux coming from distant sources, allowing us to study objects that would otherwise be too faint or too small to be resolved with the current instrumentation. In this thesis, I exploit both these aspects, to achieve three goals: i) measure the total mass profiles of extremely massive early-type galaxies that act as lenses at intermediate redshift, and employ them to measure the values of the cosmological parameters; ii) characterize the stellar population of the star-forming clumps that we observe in high-z, lensed galaxies; and iii) employ the Lyman-alpha (Lya) emission from distant galaxies to constrain the geometry of the circumgalactic medium surrounding typical high-z galaxies.