Latest Observations of M31 with the Fermi Large Area Telescope: A Galactic Center Excess in Andromeda?

The Fermi LAT has opened the way for comparative studies of cosmic rays (CRs) and high-energy objects in the Milky Way (MW) and in other, external, star-forming galaxies. We revisited the gamma-ray emission in the direction of M31 using more than 7 yr of LAT Pass 8 data in the energy range 0.1-100 GeV. M31 is detected with a significance of nearly 10 sigma and the source is observed to be extended with a 4 sigma significance. Its spectrum is consistent with a power law. The spatial distribution of the emission is consistent with a uniform brightness disk over the plane of sky and no offset from the center of M31, but nonuniform intensity distributions cannot be excluded. The flux from M31 appears confined to the inner regions of the galaxy and does not fill the plane of the galaxy or extend far from it. The gamma-ray signal is not correlated with regions rich in gas or star-formation activity suggesting that the emission is not interstellar in origin, unless the energetic particles radiating in gamma rays do not originate in recent star formation. Alternative and nonexclusive interpretations are that the emission results from a population of millisecond pulsars dispersed in the bulge and disk of M31 by disrupted globular clusters or from the decay or annihilation of dark matter particles, similar to what has been proposed to account for the so-called Galactic center excess found in Fermi-LAT observations of the MW.