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

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 4.7$\sigma$ significance for a $0.9^{\circ}$ (12 kpc) halo with a photon flux of $\sim (3.2\pm1.0)\times 10^{-9} ~\mathrm{cm^{-2}s^{-1}}$ and a luminosity of $(4.0\pm1.5)\times 10^{38} ~\mathrm{erg~s^{-1}}$ 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 $(3.3 \pm 1.0) \times 10^{-10}~\mathrm{cm^{-2}s^{-1}}$. The corresponding gamma-ray luminosity, $5\times10^{37} ~\mathrm{erg~s^{-1}}$ 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.