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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. Using 2 yr of observations with the Fermi LAT, localGroup galaxy M31 was detected as a marginally extended gamma-ray source, whileonly an upper limit (UL) has been derived for the other nearby galaxy M33. Werevisited the gamma-ray emission in the direction of M31 and M33 using morethan 7 yr of LAT Pass 8 data in the energy range 0.1-100 GeV, presentingdetailed morphological and spectral analyses. M33 remains undetected and wecomputed an UL for it. This revised UL remains consistent with the observedcorrelation between gamma-ray luminosity and star-formation rate tracers andimplies an average CR density in M33 that is at most half of that of the MW.M31 is detected with a significance of nearly 10 sigma and to be extended with4 sigma. Its spectrum is consistent with a power law. The spatial distributionof the emission is consistent with a uniform disk with a radius of 0.4 deg andno offset from the center of M31, but nonuniform intensity distributions cannotbe excluded. The flux from M31 appears confined to the inner regions of thegalaxy and does not fill the disk or extend far from it. The gamma-ray signalis not correlated with regions rich in gas or star-formation activitysuggesting that the emission is not interstellar in origin, unless theenergetic particles radiating in gamma rays do not originate in recent starformation. Alternative and nonexclusive interpretations are that the emissionresults from a population of millisecond pulsars dispersed in the bulge anddisk of M31 by disrupted globular clusters or from the decay or annihilation ofdark matter particles, similar to what has been proposed to account for theso-called Galactic Center excess found in Fermi-LAT observations of the MW.

Observations of M31 and M33 with the Fermi Large Area Telescope: a galactic center excess in Andromeda?