Giant cosmic ray halos around M31 and the Milky Way

Understand

Recently, a diffuse emission of 1-100 GeV $\gamma$-rays has been detected from the direction of Andromeda.

• The emission is centered on the galaxy, and extends for $\sim 100-200$ kpc away from its center.
• Explaining the extended $\gamma-$ray emission within the framework of standard scenarios for the escape of cosmic rays injected in the galactic disk or in the galactic center is problematic.
• In this paper, we argue that a cosmic ray origin (either leptonic or hadronic) of the $\gamma$-ray emission is possible in the framework of non standard cosmic ray propagation scenarios or in the case of particle acceleration taking place in the galaxy's halo.