Probing the Spatial and Spectral Distribution of Galactic Cosmic Rays with High-Energy Gamma-Rays

Gamma-ray astronomy provides a unique tool to study Galactic cosmic rays far from Earth. Cosmic rays in fact, during their propagation, interact with the interstellar medium, producing high-energy gamma rays that can be detected from Earth. These gamma rays carry the information on the origin and the energetics of the parent cosmic rays at the location of interaction. The detection is facilitated in regions with enhanced gas density, like giant molecular clouds, or in regions with enhanced cosmic-ray flux, for example near an accelerator. In this thesis, I studied both scenarios by analyzing the data accumulated by the Fermi Large Area Telescope between a few tens of MeV up to almost 1 TeV. Firstly,the gamma-ray emission arising from giant molecular clouds, distributed all over the Milky Way, has been investigated, obtaining an unprecedented mapping of the cosmic-ray distribution in the Galaxy. Secondly, the emission that originates in the medium surrounding the supernova remnant W44 has been studied, confirming the presence of recently accelerated particles and unveiling evidence for anisotropic escape. Finally, the potential of the current and future gamma-ray instruments in detecting the radiation emitted from the interstellar medium, both in correspondence of clouds and in the vicinity of accelerators, is evaluated and discussed