Gamma-rays from the circumgalactic medium of M31

Abstract

ABSTRACT We discuss the production of γ-rays from cosmic rays (CR) in the circumgalactic medium (CGM) of Andromeda (M31) in light of the recent detection of γ-rays from an annular region of ∼5.5–120 kpc away from the M31 disc. We consider the CRs accelerated as a result of the star formation in the M31 disc, which are lifted to the CGM by advection due to outflow and CR diffusion. The advection time-scale due to bulk flow of gas triggered by star formation activity in the M31 disc is comparable (∼Gyr) to the diffusion time-scale with diffusion coefficient ≥1029 cm2 s−1 for the propagation of CR protons with energy ∼412 GeV that are responsible for the highest energy photons observed. We show that a leptonic origin of the γ-rays from cosmic ray (CR) electrons has difficulties, as the inverse Compton time-scale (∼Myr) is much lower than advection time-scale (∼Gyr) to reach 120 kpc. Invoking CR electrons accelerated by accretion shocks in the CGM at ∼100–120 kpc does not help since it would lead to diffuse X-ray features that are not observed. We, therefore, study the production of γ-rays via hadronic interaction between CR protons and CGM gas with the help of numerical two-fluid (thermal + CR) hydrodynamical simulation. We find that a combination of these mechanisms that are related to the star formation processes in M31 in the last ∼Gyr, along with diffusion and hadronic interaction, can explain the observed flux from the CGM of M31.