Galactic bulge preferred over dark matter for the Galactic centre gamma-ray excess

Abstract

An anomalous gamma-ray excess emission has been found in the Fermi Large Area Telescope data 1 covering the centre of the Galaxy 2,3 . Several theories have been proposed for this 'Galactic centre excess'. They include self-annihilation of dark-matter particles 4, an unresolved population of millisecond pulsars 5, an unresolved population of young pulsars 6, or a series of burst events 7 . Here, we report on an analysis that exploits hydrodynamical modelling to register the position of interstellar gas associated with diffuse Galactic gamma-ray emission. We find evidence that the Galactic centre excess gamma rays are statistically better described by the stellar over-density in the Galactic bulge and the nuclear stellar bulge, rather than a spherical excess. Given its non-spherical nature, we argue that the Galactic centre excess is not a dark-matter phenomenon but rather associated with the stellar population of the Galactic bulge and the nuclear bulge.