Gamma rays as probes of cosmic-ray propagation and interactions in galaxies

TL;DR

Gamma-ray observations serve as a crucial tool for understanding cosmic-ray propagation and interactions across various galactic environments, revealing detailed insights into non-thermal particle populations and challenging existing models.

Contribution

This paper reviews recent advances in gamma-ray observations and their impact on understanding cosmic-ray life cycles and interactions in galaxies.

Findings

Gamma-ray data reveal detailed properties of galactic cosmic rays.

Observations challenge standard assumptions in cosmic-ray modeling.

Progress in data analysis methods enhances understanding of cosmic-ray processes.

Abstract

Continuum gamma-ray emission produced by interactions of cosmic rays with interstellar matter and radiation fields is a probe of non-thermal particle populations in galaxies. After decades of continuous improvements in experimental techniques and an ever-increasing sky and energy coverage, gamma-ray observations reveal in unprecedented detail the properties of galactic cosmic rays. A variety of scales and environments are now accessible to us, from the local interstellar medium near the Sun and the vicinity of cosmic-ray accelerators, out to the Milky Way at large and beyond, with a growing number of gamma-ray emitting star-forming galaxies. Gamma-ray observations have been pushing forward our understanding of the life cycle of cosmic rays in galaxies and, combined with advances in related domains, they have been challenging standard assumptions in the field and have spurred new developments in modelling approaches and data analysis methods. We provide a review of the status of the subject and discuss perspectives on future progress.