Gamma rays from dense interstellar clouds.

The recent COS-B observations of the high energy (greater than 300 MeV) gamma-ray emission of the galactic disk have related in the longitude (II) range between 10 and 40 degrees a component at low positive latitudes, prominent in the longitude (II) range between 25 and 35 degrees. The position of this excess coincides with large dark clouds in the Lynds survey. In this paper, whether a diffuse production of gamma rays in the close-by interstellar medium can account for the observed excess emission is investigated. Both nuclear interactions of cosmic-ray nuclei with the interstellar gas and bremsstrahlung of cosmic-ray electrons are considered as the main emission mechanisms. The neutral atomic hydrogen column density detected in the region of the gamma-ray excess clearly falls short of accounting for the gamma-ray brightness. On the other hand, the gamma-ray production in the observed dark nebulae leads to a total emission compatible with the COS-B observations. Using recent estimates of the gamma-ray production yield, such a result shows that local dense clouds are completely penetrated by cosmic rays.