The role of gamma-ray in relation to the observed diffuse emission and the cosmic ray origin problem

Abstract

Interpretation of the present gamma-ray data above 100 MeV is discussed in relation to cosmic ray interactions with the atomic and molecular hydrogen in the Galactic plane. The SAS II gamma-ray data are analysed for supportive evidence on the 2CG candidate sources identified from the COS B observations. The strongest sources are confirmed. There is good evidence to suggest that many of the weaker sources are not truly discrete. A Monte-Carlo analysis of the discrete source detection efficiencies suggests that many 2CG sources are unresolved giant molecular clouds, irradiated by the ambient cosmic ray flux. An attempt is made to define a genuine source catalogue. Taking account of the detection efficiencies the net source flux (from both resolved and unresolved sources) is estimated to be 11-23% of the Galactic plane emission. Cosmic ray interactions with the Orion molecular cloud complex are investigated through an analysis of the gamma-ray emission above 100 MeV. There is no evidence for cosmic ray exclusion from the clouds. A new derivation of the CO/H (_2) ratio is obtained. N (_H2) / T ((^12) CO) dv = (3.7 ± 0.6) x 10(^20) at cm (^-2) (K kms (^-1)) (^-1) Contributions to the extragalactic gamma-ray flux from radio galaxies and rich clusters are estimated. The flux may be dominated by emission from these objects. With the estimate of the discrete Galactic source flux and the CO→H (_2) ratio, the radial gamma-ray emissivity is compared to the HI, inferred H (_2) and possible cosmic ray radial densities in the inner Galaxy. It is shown that a moderate cosmic ray gradient overestimates the gamma-ray flux, unless the metal abundance gradient reduces the inferred H (_2) mass.