Shedding light on the anomalous lens JVAS B1555+375 with global-VLBI

The ΛCDM model is able to describe the large scale structure of the Universe, but it shows inconsistencies at galactic/sub-galactic scales. This is exemplified by the so-called "missing satellite problem", where only a few satellites are observed in the Local Group, despite the large number of sub-halos predicted by simulations. To test this discrepancy at high redshift, the only tool is provided by strong gravitational lensing. Strong lensing can probe the sub-halos presence by means of their gravitational effect, which is expected to be at milliarcsecond scale. Some gravitational systems show deviations from the predictions of a smooth model, known as anomalies, which are often attributed to the presence of sub-halos and used to statistically test the predictions of the ΛCDM model. In this thesis we present a study of the lensing system JVAS B1555+375 with a global-VLBI experiment at 18 cm at milliarcsecond angular resolution. This system shows a strong flux ratio anomaly and near-infrared observations revealed the presence of an edge-on disc in the lensing galaxy, which crosses over the pair of images exhibiting the anomaly. Our high resolution and high sensitivity observation revealed for the first time the presence of an astrometric anomaly in the same pair and the detection of only three of the four predicted images. The structure of the background object was resolved in two of these three images and is compatible with a jetted AGN. We can explain the anomalies in the system by modelling the lens with an isothermal ellipsoid, an exponential disc, and without requiring the presence of a dark matter sub-halo. We conclude that the baryonic ionized matter of the edge-on disc is the dominant responsible for the anomalies. Our study confirms that not all lensing anomalies are due to dark matter sub-halos and not accounting for the full complexity of the lenses may overestimate the amount of substructures in distant lensing galaxies predicted by statistical studies.