  The Andromeda Galaxy is the closest spiral galaxy to us and has been the
subject of numerous studies. It harbors a massive dark matter (DM) halo which
may 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 an
M31-Milky Way (MW) distance of 785 kpc, but its presumably low surface
brightness makes it challenging to detect with gamma-ray telescopes. Using 7.6
years of Fermi Large Area Telescope (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 perform an
in-depth analysis of the systematic uncertainties related to the observations.
We use the cosmic ray (CR) propagation code GALPROP to construct specialized
interstellar emission models (IEMs) to characterize the foreground gamma-ray
emission from the MW, including a self-consistent determination of the
isotropic component. We find evidence for an extended excess that appears to be
distinct from the conventional MW foreground, having a total radial extension
upwards of ~120-200 kpc from the center of M31. We discuss plausible
interpretations of the excess emission but emphasize that uncertainties in the
MW foreground, and in particular, modeling of the H I-related components, have
not been fully explored and may impact the results. This study was first
presented in a poster at the 8th International Fermi Symposium. The article
describing the full analysis is under preparation and will be published
elsewhere.
