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Published October 1, 1975 | Published
Journal Article Open

Molecular hydrogen in the Galaxy and galactic gamma rays


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.

Additional Information

© 1975. The American Astronomical Society. Received 1975 February 5; revised 1975 March 26. We wish to acknowledge and thank Dr. Jean-Loup Puget, Dr. Giovanni Bignami, and Dr. Giovanni Fazio for stimulating and helpful discussions, Dr. Carl Fichtel for communicating the results of his SAS-2 observations prior to publication, and especially Mr. Joseph Bredekamp for programming the numerical calculations used in this paper.

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Published - 1975ApJ___201___90S.pdf


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August 19, 2023
October 20, 2023