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The Andromeda Galaxy is the closest spiral galaxy to us and has been thesubject of numerous studies. It harbors a massive dark matter (DM) halo whichmay span up to ~600 kpc across and comprises ~90% of the galaxy's total mass.This halo size translates into a large diameter of 42 degrees on the sky for anM31-Milky Way (MW) distance of 785 kpc, but its presumably low surfacebrightness makes it challenging to detect with gamma-ray telescopes. Using 7.6years of Fermi Large Area Telescope (Fermi-LAT) observations, we make adetailed study of the gamma-ray emission between 1-100 GeV towards M31's outerhalo, with a total field radius of 60 degrees centered at M31, and perform anin-depth analysis of the systematic uncertainties related to the observations.We use the cosmic ray (CR) propagation code GALPROP to construct specializedinterstellar emission models (IEMs) to characterize the foreground gamma-rayemission from the MW, including a self-consistent determination of theisotropic component. We find evidence for an extended excess that appears to bedistinct from the conventional MW foreground, having a total radial extensionupwards of ~120-200 kpc from the center of M31. We discuss plausibleinterpretations of the excess emission but emphasize that uncertainties in theMW foreground, and in particular, modeling of the H I-related components, havenot been fully explored and may impact the results.

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