Diffuse gamma radiation from the galaxy and from extragalactic sources

Abstract

Abstract From the earliest satellite measurements, it was clear that the high energy gamma ray sky was dominated by radiation from the galactic plane. The theoretical analyses of the time, and now, suggest that the majority of this radiation is diffuse, coming from cosmic ray interactions with the galactic matter and photons. The gamma ray measurements, therefore, provide the opportunity to gain significant additional insight into the galactic cosmic ray density distribution, the level of coupling of the cosmic rays and the matter, and, as it has developed, a better estimate of the molecular hydrogen density normalization. A cosmic ray density variation, at least broadly coupled to the galactic matter distribution, seems now to be supported by most of the analyses. This conclusion is consistent with the cosmic radiation being galactic and suggests the importance of future more sensitive gamma ray observations for the study of the detailed relationship between the cosmic rays and the galactic matter. An analysis of the radiation from high galactic latitudes suggest that, when the galactic radiation is subtracted, there is a residual isotropic radiation, which is believed to be extragalactic in origin. The diffuse gamma radiation from three local normal galaxies, the Small and Large Magellanic Clouds and M31, should be visible to at least one of the gamma ray telescopes on the Gamma Ray Observatory, and the first two should be seen by GAMMA-1. It is expected that the intensity and structure of both the Magellanic Clouds can be examined sufficiently well to study the cosmic ray density and thereby to determine the relevant scale of coupling for the cosmic rays and diffuse matter. Further, because of the apparently markedly different distributions of molecular and atomic hydrogen, particularly in the Small Magellanic Cloud, and the differences between the galaxies, an independent measure of the normalization of the diffuse molecular hydrogen should be possible for these two galaxies.