Diffuse galactic gamma-ray continuum emission

Some of the most important questions about the diffuse gamma-ray continuum emission from the Galaxy are reviewed, based on Compton Observatory (CGRO) results, especially COMPTEL and EGRET, and also earlier COS-B analyses. The key issues include the rôle of emission from cosmic-ray interactions with molecular hydrogen and its energy dependence, emissivity gradients and their interpretation, the cosmic-ray electron spectrum and the effect of discrete sources. The relative contribution of the various emission processes at low and high latitudes is estimated and a plausible synthesis of the observed spectrum over 5 decades of energy is presented. In the energy range above 30 MeV, models based either on explicit cosmic-ray gradients or cosmic-ray/gas coupling can give acceptable fits to the data, and a clear distinction has yet to be made. The quality of the EGRET data may make this possible in the future. The value of the CO-to-H₂ conversion factor from -rays is still uncertain and there is considerable evidence for cloud-to-cloud variations. The existence of a small emissivity gradient is well established, but is difficult to explain in a diffusive cosmic-ray propagation picture with sources distributed like SNR or pulsars unless there is a larger halo than suggested by cosmic-ray composition studies. In the energy range 1–30 MeV covered by COMPTEL the spectrum of the diffuse emission has been measured and is consistent with a combination of bremsstrahlung and inverse-Compton emission; spatial analysis shows strong evidence for a component with a wide latitude extent which is plausibly identified with the inverse-Compton component. The molecular hydrogen appears to be only a weak -ray emitter at low energies, which can be interpreted in terms of reduced MeV cosmic-ray electron density in molecular clouds. New data on the hard X-ray diffuse galactic emission is becoming available and indicates the need for a low-energy upturn in the electron spectrum or some other additional component. The contribution of unresolved sources to the diffuse emission is unknown but-probably lies in the range 10–20%. At high latitudes the galactic emission is intense enough to significantly complicate the identification of the extragalactic component; in particular the inverse-Compton emission from a halo a few kpc in extent can account for much of the high-latitude galactic emission. The detection of the Large Magellanic Cloud and the non-detection of the Small Magellanic Cloud provide constraints on extragalactic cosmic-rays, and provide an interesting comparison with the properties of the galactic system. On account of the large amount of data from CGRO now available, this is a subject in rapid development, and this paper provides a snapshot of the situation around mid-1995.