Molecular hydrogen in the Galaxy and galactic gamma rays

Abstract

Recent surveys of 2.6-cm CO emission and 100-MeV γ-radiation in the galactic plane reveal a striking correlation suggesting that both emissions may be primarily proportional to the line-of-sight column density of H_2 in the inner Galaxy. Both the γ-ray and CO data suggest a prominent ring or arm consisting of cool clouds of H_2 at a galactocentric distance of ~5 kpc with a mean total hydrogen density equivalent to ~5 atoms cm^(-3). Estimates are made of column densities of H_2 at 0° galactic longitude. The estimates are compared with estimates from infrared and X-ray absorption measurements. These estimates are all consistent, indicating that H_2 is far more abundant than H I in the inner Galaxy and is the key to a more satisfactory explanation of the γ-ray observations than previous suggestions. The importance of H_2 in understanding galactic γ-ray observations is also reflected in the correlation of galactic-latitude distribution of γ-rays and dense dust clouds. The deduced cosmic-ray distribution inferred from the calculations is similar to that of galactic supernova remnants, suggesting a galactic origin for most cosmic rays. A detailed calculation of the γ-ray flux distribution in the 0°-180° longitude range using the CO data to obtain the average distribution of molecular clouds in the Galaxy shows that most of the enhancement in the inner Galaxy is due to π^0-decay radiation and the 5-kpc ring plays a major role. Detailed agreement with the γ-ray data is obtained with the additional inclusion of contributions from bremsstrahlung and Compton radiation of secondary electrons and Compton radiation from the intense radiation field near the galactic center.

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