Cosmic rays in star-forming galaxies

The energy density of cosmic ray protons (CRp) in star-forming environments can be (i) measured from γ-ray π^o-decay emission, (ii) inferred from the measured radio non-thermal synchrotron emission (once a theoretical p/e ratio and particle-field equipartition have been assumed), and (iii) estimated from the observed supernova rate and the deduced CRp residency time. For most of the currently available galaxies where these methods can be simultaneously applied, the results of the various methods agree and suggest that CRp energy densities range from script O sign (10^-1) eV cm ^-3 in very quiet environments up to script O sign (10²) eV cm^-3 in very active ones. The only case for which the methods do not agree is the Small Magellanic Cloud, where the discrepancy between measured and estimated CRp energy density may be due to a smaller characteristic CR confinement volume.