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The Fermi Gamma-Ray Space Telescope has provided evidence for diffusegamma-ray emission in the central parts of the Milky Way and the Andromedagalaxy. This excess has been interpreted either as dark matter annihilationemission or as emission from thousands of millisecond pulsars (MSPs). We haverecently shown that old massive globular clusters may move towards the centerof the Galaxy by dynamical friction and carry within them enough MSPs toaccount for the observed gamma-ray excess. In this paper we revisit the MSPscenario for the Andromeda galaxy, by modeling the formation and disruption ofits globular cluster system. We find that our model predicts gamma-ray emission$\sim 2-3$ times larger than for the Milky Way, but still nearly an order ofmagnitude smaller than the observed Fermi excess in the Andromeda. Our MSPmodel can reproduce the observed excess only by assuming $\sim 8$ times largernumber of old clusters than inferred from galaxy scaling relations. To explainthe observations we require either that Andromeda deviates significantly fromthe scaling relations, or that a large part of its high-energy emission comesfrom additional sources.

Millisecond pulsars and the gamma-ray excess in Andromeda