Beyond Recollimation: How Shocks Ignite Instabilities in Relativistic Jets (Oltre la ricollimazione: come gli shock accendono le instabilita' nei getti relativistici)

This thesis has been conducted for my PhD in Physics at the Università degli Studi dell'Insubria, DeIpartimento di Scienza e Alta Tecnologia, in collaboration with the Istituto Nazionale di Astrofisica (INAF) - Osservatorio di Brera e Merate, under the supervision of Dr. Fabrizio Tavecchio and Dr. Gianluigi Bodo. The thesis aimed to characterize the physics of relativistic jets in Active Galactic Nuclei at recollimation sites through numerical simulations. Recollimation is a region of particular importance in the dynamical evolution of jets and in the emission of high-energy radiation. Additionally, it provides insights into fundamental properties of jets in the vicinity of the AGN core. Three-dimensional (magneto-)hydrodynamic simulations revealed that jet conditions at recollimation favor the growth of disruptive instabilities, challenging the traditional view—supported by two-dimensional simulations—of confined jets undergoing a series of recollimation and reflection shocks. Motivated by the implications of this alternative scenario, I investigated the stability of relativistic AGN jets at recollimation. The core of this work involved a parameter study on the evolution of instabilities in astrophysical jets and their effect on jet thermodynamics. Specifically, I performed two-dimensional and three-dimensional hydrodynamic simulations using the state-of-the-art PLUTO code. These simulations, carried out on the CINECA and PLEIADI (INAF cluster) supercomputing facilities, required proposals for computational time allocation. The results provide valuable insights into the complex processes governing AGN jets and can be used to model the properties of low-power, radio-loud AGNs. As a second part of the project, I applied these findings to propose a scenario for the deceleration and radio emission of jets in a newly discovered class of faint Fanaroff-Riley type 0 (FR0) radio galaxies.