Millisecond Pulsars and the Gamma-Ray Excess in Andromeda

The Fermi Gamma-Ray Space Telescope has provided evidence for diffuse gamma-ray emission in the central parts of the Milky Way and the Andromeda galaxy. This excess has been interpreted either as dark-matter annihilation emission or as emission from thousands of millisecond pulsars (MSPs). We have recently shown that old massive globular clusters (GCs) may move toward the center of the Galaxy by dynamical friction and carry within them enough MSPs to account for the observed gamma-ray excess. In this Letter we revisit the MSP scenario for the Andromeda galaxy by modeling the formation and disruption of its GC system. We find that our model predicts gamma-ray emission ∼2-3 times larger than for the Milky Way, but still nearly an order of magnitude smaller than the observed Fermi excess in the Andromeda. Our MSP model can reproduce the observed excess only by assuming ∼8 times a larger number of old clusters than inferred from galaxy scaling relations. To explain the observations we require either that Andromeda deviates significantly from the scaling relations, or that a large part of its high-energy emission comes from additional sources.