High Energy Emission in Relativistic Jets of AGN: Theory predictions and gamma-ray view

The last decade has been a golden epoch for the observations of extra-galactic jets. The extension of the observational window from the radio through infrared and optical up to the high energy band has provided us with a wealth of new data. The study of the jets by modeling their spectral energy distribution has become a very effective approach. Multi-band observations are determinant to constrain the model parameters and probe the radiative environment where the jets form and expand. The broadband fitting allows us to infer the main physical parameters of the emitting plasma and to consequently estimate the total jet kinetic power. In this thesis a simple leptonic synchrotron and inverse Compton model is adopted and generalized by taking into account the main local and external radiative fields which can act as seed photons. The model is used to derive constraints on the jet contribution to the total high energy emission in compact radio sources. These are supposed to be the young counterparts of the giant radio sources. Unveiling the origin of their high energy emission is crucial to understand their subsequent evolution and the nature of the feedback mechanism with the intergalactic medium. First, we discuss the modeled broadband emission of jets in low and high power young radio sources observed at increasing viewing angles. The role of the seed photons is investigated for different jet velocities and linear sizes. We then test the model on the spectral energy distribution of the compact quasar 3C 186. The presence of a velocity structure internal to the jet appears to be a crucial requirement to ensure its radiative relevance at the high energies. The internal structure of the jet is the subject of a dedicated study carried out on the FR I radio galaxy NGC 6251. The two radiative models used to describe its non-thermal broadband nuclear mission return back two deeply different pictures: a heavy and slow jet for the synchrotron self Compton model while a light but highly dissipative one for the structured jet model (spine-layer).