Fermi Gamma-ray Space Telescope

Fermi-LAT Observations of Gamma-ray Emission Towards the Outer Halo of M31

Chris Karwin
(Simona Murgia, Sheldon Campbell, Igor Moskalenko)


The outer halo of M31 has a radial extension of ~300 kpc, which corresponds to a diameter of 42 degrees on the sky for an M31--Milky Way (MW) distance of 785 kpc. Gamma-ray emission from M31's outer halo may arise from the interaction of high energy cosmic-rays (CRs) with the radiation field of the stellar halo and/or the circumgalactic gas. Additionally, it may arise from more exotic processes such as dark matter (DM) annihilation or decay. Using 7.6 years of Fermi-LAT observations, we make a detailed study of the gamma-ray emission between 1--100 GeV towards M31's outer halo, with a total field radius of 60 degrees centered at M31, and we perform an in-depth analysis of the systematic uncertainties related to the observations. We find evidence for a spherically symmetric excess that appears to be distinct from the conventional MW foreground. We discuss plausible interpretations for the excess emission, but emphasize that uncertainties in the MW foreground, and in particular modeling of the HI-related components, have not been fully explored and strongly impact the results. Our observations and interpretation are presented.