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 (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 • 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 towards 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 (CR) propagation code GALPROP to construct specialized interstellar emission models (IEMs) 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 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.

Exploring a cosmic-ray origin of the multiwavelength emission in M31

“…In addition to gamma rays produced from IC scattering, we also consider gamma rays resulting from the decay of neutral pions produced in cosmic-ray proton collisions. When the pions are produced they decay rapidly within a time span of ∼ 10 −16 s. The gamma rays do not experience diffusion or radiative loss effects, and thus we do not need to consider equation (7). Instead, for a π 0 gamma-ray source injection Q γ (in units of GeV −1 cm −3 s −1 ), the flux is simply given by integrating over the volume of the source [27,64,79]:…”

“…The Andromeda galaxy (M31) is particularly enticing as a target of gamma-ray studies due its status as the nearest large spiral galaxy. M31 has been the focus of several previous gamma-ray searches [1][2][3][4][5][6][7][8]. Early observations [1][2][3][4] were only able to place upper limits until the galaxy was first detected in gamma-rays using 2 years of Fermi-LAT data at 5.3σ significance, along with some evidence of a spatial extension at the 1.8σ confidence level [5].…”

The Self-Control of Cosmic Rays

“…The neutrino flux compares well with the observations of IceCube, provided there is an overdensity of ∼ 100 − 200 in the gas, compatible with what is expected inside the virial radius of a structure such as our Galaxy. Interestingly, a diffuse gamma ray emission has recently been measured by Fermi-LAT from a large region around the Andromeda galaxy [66]. Whether such emission is due to the phenomenon described here or to a CR driven wind (see §2), or to some yet unknown phenomenon remains to be seen, based on additional investigation that is currently ongoing.…”

Gamma rays: direct observations

“…In [17], after selecting a tuning region embracing the region of interest and subtending approximately the same galactic location, the authors proceed as described in the previous section, analyzing 7.6 years of Fermi-LAT data until residuals are under control. Applying the derived diffuse emission model to the M31 region they report a 3-5% residual around 10 GeV, which does not disappear when tweaking the interstellar emission model.…”