The distribution of atomic and molecular hydrogen in the Galactic plane

An examination is made of the radial distribution of atomic and molecular hydrogen in the plane of the Galaxy. The mass of atomic hydrogen in the outer Galaxy is found to be much larger than previously expected. The large-scale distribution of molecular hydrogen cannot be observed directly but must be inferred from observations of the CO molecule, a procedure involving many uncertainties. The conversion from CO to H2 densities derived in this work is regarded as giving a lower limit to the molecular density. The gas densities derived are shown to produce a consistent picture with observations of cosmic #betta#-rays when a cosmic-ray intensity distribution derived from radio synchrotron measurements is used. The mass of molecular hydrogen in the inner Galaxy is shown to be approximately the same as that of atomic hydrogen. The surface density of molecular hydrogen between 12 and 22 kpc from the Galactic Centre is shown to be about 18 per cent of that of atomic hydrogen. (author)