UC Berkeley

Although negligible by number density, relativistic cosmic rays (CRs) are an energetically important component of the plasmas filling the inter-stellar media and halos of galaxies, and the cores of galaxy clusters. As a result, CRs can have a large impact on their host systems, for example by heating diffuse gas or by driving galactic winds. This "CR feedback" is a strong function of how CRs are transported across galactic environments. Despite elaborate efforts to constrain it, the transport of galactic CRs remains uncertain, which makes CR feedback one of the biggest unsolved puzzles in galaxy evolution. The goal of this thesis has been two-fold. A large portion of this work uses theoretically favoured models of CR transport to study how CRs affect the dilute gases in galactic halos and in the intra-cluster medium of galaxy clusters. It is shown that CRs can significantly affect wave propagation by driving rapidly growing fluid instabilities, which has significant implications for the thermal evolution of these systems. This work is complemented by a study that compares existing micro-physical theories of CR transport with detailed measurements of spectra of galactic CRs in the solar neighbourhood. It reveals that existing CR transport models are generally not in good agreement with the data and provides speculations on how this discrepancy could be remedied.