Diffuse gamma-ray emission in the galactic plane from cosmic-ray, matter, and photon interactions

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On the basis of the spatial distribution, intensity, and energy spectrum, the diffuse Galactic high-energy gamma radiation is believed to be the result of cosmic-ray interactions with matter, and to a lesser extent photons. This paper describes a model calculation of diffuse gamma-ray emission based on these interactions. Recent radio observations of the main interstellar components of matter on a scale of 0°.5 are used for the entire region within 10° of the Galactic plane. A three-dimensional spatial model of the Galaxy is used to compute the emission from each volume element based on the matter, cosmic ray, and photon densities using well know interaction processes. Provisions are incorporated to account for the near-far ambiguity in determining the distance of matter in the inner Galaxy that results from Galactic rotation velocity-distance relationship. Cosmic-ray densities are modeled from the matter distribution using dynamical balance arguments, and they are expressed as a coupling scale length. This length, together with the normalization factor used to convert the observed CO line intensity to the molecular hydrogen density, are the only two adjustable parameters in the model. This approach provides a framework for calculating the observed intensity without invoking symmetries in longitude which can dilute spatial differences related to interesting spiral arm features, and it provides for a wide range of assumptions regarding the cosmic-ray distribution to be readily tested. The calculations are compared with existing data, and the values of the parameters are given. It is expected that new results from the EGRET instrument, when used with this model, will contribute to a better understanding of the cosmic ray distribution and the relation between CO and atomic hydrogen. Both of these issues are significant for studies of the dynamics and structure of the Galaxy.

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Astrophysical Journal

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