Journal
ASTROPHYSICAL JOURNAL
Volume 880, Issue 2, Pages -Publisher
IOP PUBLISHING LTD
DOI: 10.3847/1538-4357/ab2880
Keywords
astroparticle physics; cosmic rays; dark matter; galaxies: individual (M31); Galaxy: halo; gamma rays: diffuse background
Categories
Funding
- Department of Energy [DESC0014431]
- National Science Foundation [PHY-1620638]
- McCue Fellowship
- NASA [NNX17AB48G]
- National Aeronautics and Space Administration in the United States
- Department of Energy in the United States
- Commissariat a l'Energie Atomique in France
- Centre National de la Recherche Scientifique/Institut National de Physique Nucleaire et de Physique des Particules in France
- Agenzia Spaziale Italiana in Italy
- Istituto Nazionale di Fisica Nucleare in Italy
- Ministry of Education, Culture, Sports, Science, and Technology (MEXT)
- High Energy Accelerator Research Organization (KEK) in Japan
- Japan Aerospace Exploration Agency (JAXA) in Japan
- K..A.. Wallenberg Foundation in Sweden
- Swedish Research Council in Sweden
- Swedish National Space Board in Sweden
- Istituto Nazionale di Astrofisica in Italy
- Centre National d'Etudes Spatiales in France
- DOE [DE-AC02-76SF00515]
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The Andromeda galaxy is the closest spiral galaxy to us and has been the subject of numerous studies. It harbors a massive dark matter halo, which may span up to similar to 600 kpc across and comprises similar to 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 yr of Fermi Large Area Telescope (Fermi-LAT) observations, we make a detailed study of the gamma-ray emission between 1-100 GeV toward 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 propagation code GALPROP to construct specialized interstellar emission models 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 upward of similar to 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.
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