The formation of mega halos in numerical simulations

Galaxy cluster are the largest virialised structure in the Universe. The medium that fills the space between galaxies in the cluster has been observed both in X-ray and radio bands, meaning that we can observe both thermal and non-thermal processes. The non thermal processes are mostly dominated by inverse Compton scattering and synchrotron emission. The latter is particularly well observed as it produces halos that extent for not more than ∼ 1 Mpc, in diameter, detected in the majority of massive clusters with ongoing merging activity. In the recent work of Cuciti et al. (2022), the discovery of a new family of halos was reported: the mega radio halos (MHs) that can fill a volume 30 times larger than the classical radio halo regions known so far. This emission is thought to be generated from relativistic electrons through Fermi II turbulent re-acceleration processes. In this Thesis, I present the results of the first dedicated analysis of this process with new cosmological simulations, as a way of testing the turbulent re-acceleration scenario for mega radio halos. I analyse the data acquired from a cosmological, adaptive mesh refinement, ENZO simulation (Bryan et al. 2014) where the gas evolution has been studied with Crater (Wittor et al. 2016), a Lagrangian code that evolve gas tracers injected, in post-process, in the simulation. The final cluster of galaxies has a mass of M100 = 3.8 · 1014 M⊙ and a virial radius of R100 = 1.52 Mpc at z = 0. This is a low mass cluster and it undergoes a few mergers during its evolution. This system allowed me to test the possible formation scenario of mega radio halos in a typical clusters of galaxies. After a spatial evolution examination, I find that the electrons which end up in the MH region have spent a part of their evolution in the center of the cluster and shared similar physical conditions of the gas that finally fills the volume of classical radio halos. I measure that continuous accretions of massive satellites re-ignites the turbulent process maintaining the electrons at very high energy. A conspicuous fraction of the cosmic rays has been re-accelerated to reach a large enough energy to be radio emitting for synchrotron radiation. Moreover, the shape of the radial distribution of the fraction of radio emitting electrons seems to reproduce the one retrieved from the observations, indicating a plateau, in the last half Gyr of the simulation, for the whole MH region. Very interestingly, my analysis shows that the median values for the energy of populations of electrons injected at energies of GeV can be maintained at this level by the prolonged effect of turbulent re-acceleration, for over ∼ 5 Gyr, i.e. much beyond the classical view in which turbulent re-acceleration in the ICM can only act intermittently, and boost the energy of radio emitting electrons for less than ∼ 1 Gyr. Although preliminary and limited to a single simulation for the moment, these results are promising and have been presented in a letter to Astronomy & Astrophysics (Beduzzi et al., 2023, arXiv:2306.03764).