Abstract
The galaxy center gamma ray excess in the Milky Way (MW) and the Andromeda (M31) detected by Fermi-LAT has raised a debate over its origin being from the proposed dark matter (DM) annihilation or an unresolved population of millisecond pulsars (MSPs). Resolving this problem shall have great influence on where to hunt DM. We have used a new tested model of globular cluster (GC) formation to simulate the luminosity contribution from MSPs, and found that compared with a previous study with a crude GC formation model, the result 1)fits the MW excess even better , but 2) likewaise substantially insufficient to the M31 excess. This suggests that the two galaxies are more different than they were believed, that the DM distribution and the composition at the nucleus shows discernable discrepancies. However we cannot rule out the possibility that it might be due to our lesser knowledge about the gamma ray sources in M31 than the MW. Furthermore, in carrying out the work we have developed a complete pipeline of GC evolution and mass deposition, which can serve to study its contribution to the formation and properties of the nuclear star cluster (NSC). Our database stores samples of halo masses from 1e9~1e14 msun with different merging histories. Thus we are able to look at the statistics across a large span. We hope to be able to shed lights on various properties of the NSCs such as the metalicity, mass, radius etc.
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