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Bloemen, J.B.G.M., A.W. Strong, L. Blitz, R.S. Cohen, T.M. Dame, D.A. Grabelsky, W. Hermsen, F. Lebrun, H.A. Mayer-Hasselwander, and P. Thaddeus, 1986: The radial distribution of galactic gamma rays: III. The distribution of cosmic rays in the Galaxy and the CO-H
High-energy (> 70 MeV) gamma-ray observations are compared to HI and CO surveys over more than half of the Milky Way. The kinematics of both HI and CO are used as a distance indicator to determine, in combination with COS-B gamma-ray data, the galacto-centric distribution of the gamma-ray emissivity (the production rate per H atom) for three gamma-ray energy intervals. The ratio between H2 column density and integrated CO line intensity is calibrated independently of excitation and abundance effects. The galacto-centric distributions of cosmic-ray electrons and nuclei are derived separately from the gamma-ray emissivity distributions.
For each energy range the gamma-ray emissivity increases towards the inner parts of the Galaxy, but the near constancy of the high-energy (300 MeV-5 GeV) emissivity beyond the solar circle out to large distances [discussed in Paper II (Bloemen et al., 1984c)I remains valid; the gradient is strongest for low energies (70-150 MeV). The corresponding galacto-centric cosmic-ray distributions are shown to be described satisfactorily by exponential distributions for R ~> 3 kpc (R☉ = 10 kpc), with a radial scale length of 4-11 kpc for electrons (with energies up to several hundreds of MeV) and a scale length > 18 kpc for nuclei (with energies of a few GeV). Although a cosmic-ray electron gradient is required to explain the observations, the results are consistent with a constant density of cosmic-ray nuclei throughout the entire Galaxy. These cosmic-ray gradients are upper limits if a population of unresolved galactic gamma-ray sources exists with a latitude distribution similar to that of the gas, but with a stronger concentration towards the inner parts of the Galaxy. Previous studies that indicated a strong density gradient for cosmic-ray nuclei in the outer Galaxy, first claimed by Dodds et al. (1975), are shown to be incorrect.
On a large scale, the ratio between molecular-hydrogen column density and integrated CO line intensity is found to be constant throughout the Galaxy, within uncertainties. Our estimate of this ratio is N(H2)/WCO = 2.8×l020 mol.s/cm2K.km, although, due to systematic effects, the true value is likely to be lower. The resultant H2 mass is found to be equal to the HI mass for 2 kpc < R < 10 kpc.
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