Testing the dark matter hypothesis for the Fermi GeV-excess versus alternative explanations (molecular clouds, milli-second pulsars)

The so-called "GeV-excess'' of the diffuse Galactic gamma-ray emission, is studied with a spectral template fit based on energy spectra for each relevant process of gamma-ray emission, which allows to determine simultaneously the standard background processes and possible new signals in each sky direction. The excess can be explained by the contribution of a new source with a spectrum peaking at 2 GeV. Three sources have been proposed in the literature: a dark matter annihilation signal (DM), a signal from milli-second pulsars (MSP) and a signal from molecular clouds (MC). All three sources have spectra peaking at 2 GeV, but slightly different spectral shapes and will be compared with the data. All hypotheses provide acceptable fits, if one considers a limited field-of-view around the Galactic center. However, if one considers the whole gamma-ray sky and includes gamma-ray spectra up to 500 GeV we find that the MC hypothesis is strongly preferred over the other hypotheses for several reasons: i) The MC hypothesis provides significantly better fits; ii) The morphology of the "GeV-excess'' follows the morphology of the CO-maps, a tracer of MCs; iii) The massive central molecular zone shows the excess in its rectangular field-of-view which contradicts the spherical morphology expected for the other hypotheses.