Title: Fermi -LAT Observations of $γ$-Ray Emission toward the Outer Halo of M31

Abstract

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 ~600 kpc across and comprises ~90% of the galaxy's total mass. This halo size translates into a large diameter of 42° 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 $γ$-ray telescopes. Using 7.6 years of Fermi Large Area Telescope ( Fermi–LAT) observations, we make a detailed study of the $γ$-ray emission between 1–100 GeV toward M31's outer halo, with a total field radius of 60° 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 $γ$-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 ~120–200 kpc from the center of M31. We discuss plausible interpretations of the excess emission, but emphasize that uncertaintiesmore » in the MW foreground—and in particular, modeling of the H I-related components—have not been fully explored and may impact the results.« less

Authors:

Karwin, Christopher M.  ^[1];

• Search DOE PAGES for author "Karwin, Christopher M."
• Search DOE PAGES for ORCID "0000-0002-6774-3111"
• Search orcid.org for ORCID "0000-0002-6774-3111"

Murgia, Simona  ^[1];

• Search DOE PAGES for author "Murgia, Simona"
• Search DOE PAGES for ORCID "0000-0003-0516-9422"
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Campbell, Sheldon ^[1]; Moskalenko, Igor V.  ^[1]

• Search DOE PAGES for author "Moskalenko, Igor V."
• Search DOE PAGES for ORCID "0000-0001-6141-458X"
• Search orcid.org for ORCID "0000-0001-6141-458X"

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+ Show Author Affiliations

1. Stanford Univ., CA (United States). Hansen Experimental Physics Lab. and Kavli Inst. for Particle Astrophysics and Cosmology

Publication Date:

2019-07-30

Research Org.:

SLAC National Accelerator Lab., Menlo Park, CA (United States)

Sponsoring Org.:

USDOE

OSTI Identifier:

1560649

Grant/Contract Number:  

AC02-76SF00515; DESC0014431; PHY-1620638; NNX17AB48G

Resource Type:

Accepted Manuscript

Journal Name:

The Astrophysical Journal (Online)

Additional Journal Information:

Journal Name: The Astrophysical Journal (Online); Journal Volume: 880; Journal Issue: 2; Journal ID: ISSN 1538-4357

Publisher:

Institute of Physics (IOP)

Country of Publication:

United States

Language:

English

Subject:

79 ASTRONOMY AND ASTROPHYSICS