Evidence of Fermi bubbles around M31

TL;DR

This paper reports the detection of gamma-ray haloes around M31, resembling Fermi bubbles, indicating past energetic activity possibly from the galaxy's central black hole or star formation.

Contribution

First evidence of Fermi bubble-like gamma-ray structures around M31, expanding understanding of galaxy halo phenomena beyond the Milky Way.

Findings

Detected gamma-ray halo with flux $2.6\pm 0.6\times 10^{-9}$ cm$^{-2}$ s$^{-1}$.

Halo consists of two bubbles 6-7.5 kpc in size, aligned perpendicular to M31's disk.

Halo suggests past activity of M31's central black hole or starburst events.

Abstract

Gamma-ray haloes can exist around galaxies due to the interaction of escaping galactic cosmic rays with the surrounding gas. We have searched for such a halo around the nearby giant spiral Andromeda galaxy M31 using almost 7 years of Fermi LAT data at energies above 300 MeV. The presence of a diffuse gamma-ray halo with total photon flux $2.6\pm 0.6\times 10^{-9}$ cm$^{-2}$~s$^{-1}$, corresponding to a luminosity (0.3-100 GeV) of $(3.2\pm 0.6)\times 10^{38}$ erg s$^{-1}$ (for a distance of 780~kpc) was found at a 5.3$\sigma$ confidence level. The halo form does not correspond to the extended baryonic HI disc of M31, as would be expected in hadronic production of gamma photons from cosmic ray interaction, nor it is spherically symmetric, as could be in the case of dark matter annihilation. The best-fit halo template corresponds to two 6-7.5 kpc bubbles symmetrically located perpendicular to the M31 galactic disc, similar to the 'Fermi bubbles' found around the Milky Way centre, which suggests the past activity of the central supermassive black hole or a star-formation burst in M31.