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.
