Molecular hydrogen in the Galaxy and galactic gamma rays

Recent surveys of 2.6-cm CO emission and 100-MeV $gamma$-radiation in the galactic plane reveal striking correlation suggesting that both emissions may be primarily proportional to the line-of-sight column density of H$sub 2$ in the inner Galaxy. Both the $gamma$-ray and CO data suggest a prominent ring or arm consisting of cool clouds of H$sub 2$ at a galactocentric distance of approx. 5 kpc with a mean total hydrogen density equivalent to approx.5 atoms cm$sup -3$. Estimates are made of column densities of H$sub 2$ at l/sup II/=0degree. The estimates at 0degree are compared with estimates from infrared and X-ray absorption measurements. These estimates are all consistent and indicate that H$sub 2$ is far more abundant than H i in the inner Galaxy and is the key to a more satisfactory explanation of the $gamma$-ray observations than previous suggestions. The importance of H$sub 2$ in understanding galactic $gamma$-ray observations is also reflected in the correlation of galactic latitude distribution of $gamma$-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. (AIP)