Acceleration and propagation of cosmic rays in high-metallicity astrophysical environments

The origin of the ultra-high energy cosmic rays (10${}^{18}$-10${}^{20}$ eV) is one of the most persistent mysteries of the contemporary astrophysics. Although the radiation of the cosmic microwave background confines the accelerators to a distance of a few hundreds of Mpc, the deflections of the particle trajectories produced by magnetic fields turns the direct identification of individual sources into a very challenging task. From the observational point of view, the anisotropy searches performed by the Pierre Auger Collaboration indicate some correlation with starburst galaxies and active galactic nuclei. The latter represent indeed the largest fraction of the GeV gamma-ray emitters detected by the Fermi satellite, revealing the existence of relativistic particles in such astrophysical sources. Gamma radiation at TeV energies from various active galaxies has also been observed, and an IceCube neutrino event has been recently suggested to be connected with a GeV flare of a blazar. In the case of starburst galaxies, the number of objects discovered at GeV energies increased from four to eight in the last eight years and more of these galaxies are expected to be detected with future generations of telescopes. Theoretical studies have also suggested that starburst galaxies and active galactic nuclei are sources of ultra-high energy cosmic rays. Processes taking place in both astrophysical sources are capable of releasing a large amount of energy, which can be converted into relativistic particles in specific situations. Additionally, the high metallicity measured in such objects indicates that indeed intermediate-mass nuclei can be accelerated therein, in agreement with the mass-composition results of the Pierre Auger Collaboration. In this work, we investigate the production of cosmic rays in three different scenarios: large-scale shocks driven by starburst superwinds, bow shocks around clumps embedded in starburst superwinds, and shocks induced by collisions of broad-line region clouds against accretion disks in active galactic nuclei. All the analyses have been performed through semi-analytical simulations, whose fiducial parameters were chosen according to the compiled information from published bibliography. Studies on specific sources have been conducted applying rigorously the constraints imposed by the multi-wavelength electromagnetic information of the objects. In all these astrophysical situations, we evaluated the feasibility of accelerating particles, calculated the energy distributions of cosmic rays, determined their maximum energies, and computed the foreseen spectral energy distributions of the non-thermal radiation. The results obtained in this thesis evince the difficulty of generating particles up to ultra-high energies under common conditions and help to constrain the characteristics of the cosmic ray accelerators of such energies. Moreover, we show the importance of disentangling the ambiguous properties of objects simultaneously classified as starbursts and active galactic nuclei, in order to clarify the origin of the cosmic rays at the highest energies. All in all, the theoretical models developed in this work might contribute to the interpretation of the observational information collected by the next generation of observatories at various wavelengths and astroparticle experiments.