Molecular hydrogen in the Galaxy and galactic gamma rays

Abstract

Recent surveys of 2.6-cm CO emission and 100-MeV y-radiation in the galactic plane reveal a striking correlation suggesting that both emissions may be primarily proportional to the line-ofsight column density of H 2 in the inner Galaxy. Both the y-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 l 11 = 0. The estimates at 0 are compared with estimates from infrared and X-ray absorption measurements. These estimates are all consistent and indicate 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 y-ray observations than previous suggestions. The importance of H 2 in understanding galactic y-ray observations is also reflected in the correlation of galactic latitude distribution of y-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.