Gamma-ray halo around the M31 galaxy as seen by the Fermi LAT

Abstract

Theories of galaxy formation predict the existence of extended gas halo around spiral galaxies. If there are 10-100 nG magnetic fields at several ten kpc distances from the galaxies, extended galactic cosmic ray (CR) haloes could also exist. Galactic CRs can interact with the tenuous hot halo gas to produce observable gamma-rays. We have performed search for a gamma-ray halo around the M31 galaxy -the closest large spiral galaxy. Our analysis of almost 7 years of the Fermi LAT data revealed the presence of a spatially extended diffuse emission excess around M31. The data can be fitted using the simplest morphology of a uniformly bright circle. The best fit gave a 4.7σ significance for a 0.9 • (12 kpc) halo with a photon flux of ∼ (3.2 ± 1.0) × 10 −9 cm −2 s −1 and a luminosity of (4.0±1.5)×10 38 erg s −1 in the energy range 0.3-100 GeV. Our results also imply a low level of the flux from the disc of the M31 galaxy (3.3±1.0)×10 −10 cm −2 s −1 . The corresponding gamma-ray luminosity, 5×10 37 erg s −1 is several times smaller than the luminosity of the Milky Way. This difference could be explained by a lower star formation rate in M31: there are less CRs and the level of the ISM turbulence is lower, which in turn leads to a shorter time of CR containment.

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

“…Evidence of a spherical γ-ray halo around M31 with a 0.9 • extension is reported in Pshirkov et al (2016a). In Pshirkov et al (2016b) the morphology of the extended γ-ray emission is reported to consists of two bubbles symmetrically located perpendicular to the M31 disk, akin to the MW Fermi bubbles.…”

“…In general, disk galaxies like M31 may be surrounded by extended CR halos (Feldmann et al 2013;Pshirkov et al 2016a). Depending on the strength of the magnetic fields in the outer galaxy, the CR halo may extend as far as a few hundred kpc from the galactic disk.…”

“…The density of CRs in the outer halo is predicted to be up to 10% of that found in the disk (Feldmann et al 2013). Disk galaxies like M31 are also surrounded by a circumgalactic medium, which is loosely defined as a halo of gas (primarily ionized hydrogen) in different phases which may extend as far as the galaxy's virial radius (Gupta et al 2012;Feldmann et al 2013;Lehner et al 2015;Pshirkov et al 2016a;Howk et al 2017). In addition, the stellar halo of M31 is observed to have an extension 50 kpc (Ibata et al 2007;McConnachie et al 2009;Mackey et al 2010).…”

VERITAS Observations of M31 (the Andromeda Galaxy)

“…Following its the initial detection [2], M 31 has appeared in both the 2FGL catalogue (J0042.5+4114 [3]) and the 3FGL catalogue (J0042.5+4117 [4]), in both cases as a point source. Analyses have also been published in [14] where they examine dark matter limits from M 31 and in [15] where evidence for γ-ray emission from a spatially extended halo around M 31 is presented. We have conducted an updated analysis of the Fermi-LAT data, using the LATAnalysisScripts and Fermi Science Tools v9r33p0 to conduct a binned (0.1 • ) analysis.…”

Cosmic-ray transport and gamma-ray emission in M31

“…The nature and origin of the emission from the central regions of M31 remain somewhat controversial: on the one hand, M31's observed gamma-ray luminosity does not significantly deviate from the expectation from the known scaling relationship between infrared and gamma-ray luminosity ported emission extends out to roughly 100-200 kpc above the plane of the galaxy, although the authors acknowledge that the emission from the "far outer halo" (at angles from M31's center 8.5 • < r < 21 • ) is likely related to mis-modeling of the significant foreground emission from the Milky Way and thus less robust than the emission from the "Spherical Halo" region at angles between 0.4 • < r < 8.5 • and than the robustly-detected inner galaxy emission at r < 0.4 • . Karwin et al (2019), while not ruling it out, argues against an extended cosmic-ray halo (Feldmann et al 2013a;Pshirkov et al 2016b) based on the radial extent, spectral shape, and intensity of the observed large-radii signal. However, as we argue below, the radial extent and intensity depend critically on assumptions on cosmic-ray diffusion outside the Galactic plane and in the halo; and the spectral shape is strongly affected by foreground Galactic emission and from the intrinsic weakness and limited statistics of the signal.…”