Evidence for a hadronic origin of the Fermi Bubbles and the Galactic Excess

TL;DR

This paper provides evidence that the Fermi Bubbles and the Galactic Center excess are of hadronic origin, linked to cosmic rays trapped in supernova remnants and dense molecular clouds, challenging dark matter explanations.

Contribution

It demonstrates a hadronic origin for the Fermi Bubbles and Galactic excess, connecting gamma-ray emission to cosmic rays in supernova remnants and molecular clouds, and refutes dark matter as the cause.

Findings

Bubble-like emission is present in the Galactic plane and halo.

The spectral shape matches cosmic rays trapped in sources.

The Galactic Center excess is an artifact of line-of-sight effects.

Abstract

The Fermi-LAT data revealed giant bubbles of emission above and below the Galactic plane with an energy spectrum significantly harder than seen from other directions. How the bubbles connect to the Galactic plane is unclear. We find that bubble-like emission is not only found in the halo, but it is strongly present in the Galactic plane as well with a morphology close to the spatial distribution of the 1.8 MeV gamma-ray line from 26Al, a radioactive nucleus synthesized in SNRs. In addition, the spectral shape of this hard component coincides with the predicted spectrum from cosmic rays trapped in sources. Hence, we propose that the bubble-like emission in the plane has a hadronic origin, which arises from SCRs. The bubbles in the halo have the same energy spectrum, which suggests that they are outflows from the plane with the gamma-rays arising from hadronic interactions of protons trapped in a plasma of advected gas. Towards the Galactic center (GC) we observe the 1-3 GeV excess, but observe this excess in addition in all regions where there is strong 26Al production. This excludes the dark matter annihilation interpretation. Instead, we propose that the excess is caused by a deficit of the CRs at low rigidities in the regions of dense molecular clouds, which are characterized by higher energy losses and stronger stellar winds. The 26Al line is a tracer of such regions. If we introduce a break in the cosmic ray injection spectra in the regions of strong 26Al production to effectively take the deficit of low rigidity CRs into account, the excess in the Galactic plane and towards the GC both disappear. We thus conclude that the excess in the Galactic center is largely an artefact from the excess in the Galactic plane. The correlation between the two originates simply from the lines-of-sight towards the GC crossing the Galactic plane.