A gamma-ray beam scenario to mimic giant radio galaxies: an investigation with cosmological MHD simulations

Giant Radio Galaxies (GRG) are among the most extended structures in the Universe and are powered by supermassive black holes. Recent observations, in particular the discovery of the 7-Megaparsec long radio galaxy Porphyrion, challenge the standard interpretation of these systems as classical relativistic jets, given the large energetic demands, the apparent lack of interaction with the external medium, the absence of significant precession of their supermassive black hole, and their remarkable stability against magnetohydrodynamical instabilities. In this Thesis, I have quantitatively tested the viability of a recently proposed alternative scenario to overcome some of the aforementioned difficulties: the γ-ray beam model, in which the observed radio emission may be produced by secondary electron–positron pairs injected by high energy photons as they propagate through an intergalactic magnetic field, while interacting with the extragalactic background light. To investigate this mechanism, I developed new Julia-based numerical routines to model the propagation and radiative properties of γ-ray beams within realistic intergalactic environments, extracted from recent cosmological MHD simulations produced with the Enzo code. By statistically sampling thousands of trajectories across different redshift, I derived the expected distributions of jet sizes and compared them with the observed population of GRGs, while also predicting the corresponding radio and γ-ray luminosities and evaluating their detectability with LOFAR and Fermi-LAT. My results suggest that a fraction of the most extended radio structures may arise from this mechanism, with important implications for the interpretation of GRGs and their contribution to the magnetisation of the cosmic web.