Jet formation and propagation in the nearby radio galaxy NGC 315

Since their discovery many decades ago, relativistic jets have intrigued and fascinated astronomers. Besides being crucial elements in shaping the Universe as we observe it today, jets are unique laboratories to test and broaden our knowledge in many fields of physics, such as plasma physics and cosmology. This thesis aims at investigating the relativistic jets launched from radio galaxies, a specific class of Active Galactic Nuclei (AGN), with the goal of understanding the jet formation and propagation phenomena, as well as the connection between accretion and ejection. Specifically, the work focuses on NGC 315, a giant radio galaxy whose vicinity and large mass of the central black hole offer a unique opportunity to address these subjects. The study combines two approaches: the analysis of Very Long Baseline Interferometry (VLBI) observations is accompanied by a theoretical study carried out by means of numerical simulations. These are performed using a relativistic magnetohydrodynamic (RMHD) code. The thesis is composed of three introductory chapters followed by three results chapters. In Chapter 1, I introduce the fundamental physical and astronomical concepts concerning AGN, which are central throughout the thesis. In Chapter 2, I describe the basic elements of plasma physics and provide an overview of the state-of-the-art knowledge regarding the theory of jet formation and propagation together with the currently open questions. In Chapter 3, I explain the fundamental concepts concerning the VLBI technique, and how to perform the analysis of VLBI data. Finally, Chapters 4, 5, and 6 present the series of works on NGC 315, with Chapters 4 and 6 focusing on the observational studies and Chapter 5 describing the theoretical one. From the analysis of a multi-frequency and multi-epoch VLBI data set, the extents of the jet collimation and acceleration regions in NGC 315 are suggested to coincide. Namely, the jet is observed to collimate within ~ 0.6 pc from its apex