On the interaction between cosmic rays and dark matter molecular clouds in the Milky Way – I. Basic considerations

ABSTRA C T We explore some basic observational consequences of assuming that the dark matter in the Milky Way consists mainly of molecular clouds, and that cosmic rays can penetrate these clouds. In a favoured model of the clouds, this penetration would have the following consequences, all of which agree with observation. (i) Cosmic ray nuclei would be fragmented when they enter a cloud, giving them a lifetime in the Galaxy of ,10 15 s (for relativistic nuclei). (ii) Pionic g-rays emitted by the clouds, after proton‐proton (pp) collisions, would have a diffuse flux in the Galactic plane comparable to the flux from known sources for photon energies *1 GeV. (iii) The heat input into the clouds from cosmic rays would be re-radiated mainly in the far-infrared. The resulting radiation background agrees, in both intensity and spectrum in different directions, with a known excess in the far-infrared background of the galaxy over emission by warm dust.