The origin of cosmic rays in spiral galaxies

The problem of the origin and distribution of cosmic rays in the Galaxy is introduced by summarizing the literature on the radio and -ray studies of the Galaxy, discussing the propagation of cosmic rays in the interstellar medium, and listing the observed properties of cosmic rays. The localization of cosmic-ray electrons to their parent galaxies is an indicator that processes leading to cosmic-ray production may be common to galaxies like our own. The studies of external galaxies are therefore relevant to our own and have the advantage of better perspective.Studies of cosmic rays in exsternal galaxies are limited to the electron component which radiates synchrotron emission at radio frequencies. Multi-colour photometry of galaxies allows the separation of stellar populations that harbour particular classes of cosmic-ray sources. Statistical studies aimed at correlating integrated radio and optical properties of galaxies have reached conflicting conclusions. Although a correlation of cosmic rays with the older stellar population is proposed by some authors, others argue that the young stellar population harbours cosmic ray sources.Morphological studies of resolved galaxies provide information on the distributions of cosmic-ray electrons in galaxies. Studies in which the resolution of the radio images is much lower than in the optical are limited and have also produced contradictory results. Radio imaging at optical resolution is required for a direct comparison of cosmic-ray distributions with stellar distributions. Such studies are reviewed and the constraints they impose on cosmic-ray propagation and distribution of cosmic-ray sources is discussed.Theoretical cosmic-ray acceleration mechanisms are surveyed and an attempt is made to determine likely contributors. Mechanisms associated with shock waves in a variety of astrophysical settings are reviewed. Acceleration mechanisms not involving shocks, are also discussed. Finally, the status of the field is summarized along with some speculation on the future directions the field may take.