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Abstract

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.