Molecular clouds as the origin of the Fermi gamma-ray GeV-excess

Abstract

The Fermi-LAT data reveal an excess of diffuse gamma-rays at energies of around 2 GeV in the direction of the Galactic center. The excess has been studied by many groups and is observed above the expectation for diffuse gamma-ray emission from cosmic ray interactions with the interstellar material (π0 production from cosmic ray protons and bremstrahlung from electrons in the interstellar gas) and the interstellar radiation field (inverse Compton scattering of electrons in the interstellar radiation field). In addition to these standard components in diffuse gamma-rays we find evidence for two additional processes: π0 production in sources during acceleration and π0 production in molecular clouds. The first one is characterized by nuclear cosmic rays with a hard E-2 spectrum, expected from diffusive shockwave acceleration and can be traced by the 1.8 MeV gamma-ray line from radioactive 26 Al decays, which is synthesized in sources. The second one is characterized by nuclear cosmic rays inside molecular clouds with a sharp cutoff below 6-14 GV, which is most clearly observed in the dense Central Molecular Zone encircling the Galactic center in the Galactic disk. The cutoff leads to a suppression of low energy cosmic ray interactions in molecular clouds, which causes a shift in the maximum of the gamma-ray spectrum to higher energies, the hall-mark of the GeV-excess. This was previously interpreted as a dark matter annihilation signal. No spatial information is provided to our fit. As a result we obtain an uncorrelated and spatially highly resolved distribution of the GeV-excess. We show that a shift in the maximum of the gamma-ray spectrum, or equivalently the GeV-excess, is observed in all directions, where molecular clouds are present; these directions are available from the high resolution all-sky CO maps from the Planck satellite.