High-energy galactic gamma radiation from cosmic rays concentrated in spiral arms

A model for the emission of high-energy (>100 MeV) γ-rays from the galactic disk has been developed and compared with recent SAS-2 observations. In the calculation, it is assumed that (1) the high-energy galactic γ-rays result primarily from the interaction of cosmic rays with galactic matter, (2) on the basis of theoretical and experimental arguments the cosmic-ray density is proportional to the matter density on the scale of galactic arms, and (3) the matter in the Galaxy, atomic and molecular, is distributed in a spiral pattern consistent with density wave theory and the experimental data on the matter distribution that is available, including the 21-cm H i line emission, continuum emission from H ii regions, and data currently being used to estimate the H2 density. The calculated γ-ray distribution is in good agreement with the SAS-2 observations in relative shape and absolute flux. As a corollary, the nonuniform cosmic-ray distribution of this model tends to support the galactic origin of the fraction of the cosmic rays which are important in the production of high-energy photons. Modifications of the basis model show that the γ-ray flux is relatively sensitive to large variations of the assumed distribution of molecular hydrogen in the Galaxy. (auth)