An excess gamma-ray signal coming from the direction of the Galactic center (GC) has been detected by the Fermi Large Area Telescope (Fermi-LAT). Possible explanations for the excess include inaccuracies in the foreground/background model, an unresolved population of millisecond pulsars, and/or dark matter (DM) annihilation. However, the GC is a complicated region, and the true nature of the excess currently remains uncertain.
M31 is the closest spiral galaxy to us and has been the subject of numerous studies. Because it is both massive and relatively nearby, M31 is an ideal target for studying galaxies, and historically it has played a major role in the development of our understanding of the Universe. Gamma-ray radiation coming from the inner region of M31 was first detected in 2010 by Fermi-LAT. Since the initial detection, all studies to date have focused on the innermost region, where the galactic disk can be observed. However, M31’s galactic disk only amounts to roughly 10% of the galaxy’s total mass, with the other 90% being in the form of DM, which extends well beyond the inner region of the galaxy.
In this dissertation, a detailed study of the gamma-ray emission towards M31's outer halo is made. Using the cosmic ray propagation code GALPROP, specialized interstellar emission models are constructed to characterize the foreground emission from the Milky Way (MW), including a self-consistent determination of the isotropic component, and an in-depth analysis of the systematic uncertainties related to the observations. Evidence is found 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. A DM interpretation is found to provide a good description of the observed emission and is consistent with the GC excess DM interpretation. However, 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.