Molecular Clouds as the Origin of the Fermi Gamma-Ray GeV-Excess

Abstract

The so-called "GeV-excess" of the diffuse Galactic gamma-ray emission is studied with a spectral template fit based on energy spectra. The spectral templates can be obtained in a data-driven way from the gamma-ray data, which avoids the use of emissivity models to subtract the standardbackground processes from the data. Instead, one can determine these backgrounds simultaneously with any "signals" in any sky direction, including the Galactic disk and the Galactic center. Using the spectral template fit two hypothesis of the "GeV-excess" were tested: the dark matter (DM) hypothesis assuming the excess is caused by DM annihilation and the molecular cloud (MC) hypothesis assuming the "GeV-excess" is related to a depletion of gamma-rays below 2 GeV, as is directly observed in the Central Molecular Zone (CMZ). Both hypotheses provide acceptable fits, if one considers a limited field-of-view centered within 20${}^{\circ }$ around the Galactic center and applies cuts on the energy range and/or excludes low latitudes, cuts typically applied by the proponents of the DM hypothesis. However, if one considers the whole gamma-ray sky and includes gamma-ray energies up to 100 GeV we find that the MC hypothesis is preferred over the DM hypothesis 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, i.e. there exists a strong "GeV-excess" in the Galactic disk also at large longitudes; iii) The massive CMZ with a rectangular field-of-view of $l\times b=3.5^{\circ }\times 0.5^{\circ }$ shows the maximum of the energy flux per log bin in the diffuse gamma-ray spectrum at 2 GeV, i.e. the "GeV-excess", already in the raw data without any analysis. The rectangular profile contradicts the spherical morphology expected for DM annihilation.

53 pages, 8 figures (+ 42 figures in Appendices), extended version of arXiv:1610.08926 accepted for publication in PRD

References in corpus (10)

• Background model systematics for the Fermi GeV excess
• The Spectrum and Morphology of the Fermi Bubbles
• A Tale of Tails: Dark Matter Interpretations of the Fermi GeV Excess in Light of Background Model Systematics
• The origin of cosmic rays: Explosions of massive stars with magnetic winds and their supernova mechanism
• The Galactic ${}^{26}$Al emission map as revealed by INTEGRAL/SPI
• Distinguishing Dark Matter from Unresolved Point Sources in the Inner Galaxy with Photon Statistics
• Discovery of a New Galactic Center Excess Consistent with Upscattered Starlight
• Cosmic-Ray Models of the Ridge-Like Excess of Gamma Rays in the Galactic Center
• Fermi Bubbles and Bubble-like emission from the Galactic Plane
• The Milky Way's Central Molecular Zone

Cited by in corpus (3)

• Realistic modeling of wind and supernovae shocks in star clusters: addressing ${}^{22}\mathrm{Ne}{/}^{20}\mathrm{Ne}$ and other problems in Galactic cosmic rays
• Diffuse Galactic Gamma-rays from star clusters
• On a possible origin of the gamma-ray excess around the Galactic Center