Starburst Galaxies: A Look into the Workings of Cosmic Ray Factories

摘要

Starbursts are a new GeV and TeV source class, finally discovered after decades of predictions based on radio observations. These γ-rays, believed to be pionic emission from cosmic ray (CR) protons in the dense gas of starbursts, inform us about the bulk of the CR population. In M82 and NGC 253, magnetic fields and CRs appear to be in rough equipartition. However, CR protons appear to escape not through diffusion, but advection in the starburst wind. These starbursts also appear to be more "calorimetric" than the Milky Way, with 1/3 of the energy injected in protons lost to pion production rather than escaping. Combining the new data with radio observations sets interesting constraints on magnetic fields in starbursts (> 50μG). Starbursts and other star-forming galaxies may in fact form a major part of the unresolved cosmic GeV background, and many may be seen with the Cherenkov Telescope Array. But there are major uncertainties when interpreting the observations, such as the role of active nuclei, and the presence of emission from leptons and discrete sources like pulsars. We also consider what MeV γ-rays, including leptonic continuum and radioisotope lines, could tell us about starbursts, if they could be detected.