Observations of M31 and M33 with the Fermi Large Area Telescope: A Galactic Center Excess in Andromeda?

M Ackermann 1 M Ajello 2 A. Albert 3 L. Baldini 4 Jean Ballet 5 G. Barbiellini 6 D. Bastieri 7 R Bellazzini 8 E Bissaldi 4 E. Bottacini 9 E.D. Bloom 10 M Bregeon 11 R Bonino 12 T.J. Brandt 13 Pascal Bruel 14 R Buehler 15 R Cameron 9 R Caputo 16 M. Caragiulo 8 P. A. Caraveo 17 E. Cavazzuti 18 C. Cecchi 19 E. Charles 9 A. Chekhtman 20 G. Chiaro 21 S. Ciprini 18 F. Costanza 4 S. Cutini 18 F. d'Ammando 22 F. de Palma 4 R. Desiante 12 S. W. Digel 9 N. Di Lalla 4 M. Di Mauro 10 L. Di Venere 8 C. Favuzzi 8 S. Funk 23 P. Fusco 8 F. Gargano 4 N. Giglietto 8 F. Giordano 8 M. Giroletti 24 T. Glanzmann 10 D. Green 13 I.A. Grenier 5 Lucas Guillemot 25 S. Guiriec 13 K Hayashi 26 X. Hou 27 G. Jóhannesson 28 T. Kamae 29 J Knödlseder 30 A.K.H. Kong 31 M. Kuss 4 G. La Mura 32 S. Larsson 33 L. Latronico 12 J. Li 34 F. Longo 8 F. Loparco 11 P. Lubrano 19 S. Maldera 12 D. Malyshev, 23 A. Manfreda 4 Pierrick Martin 30 M. N. Mazziotta 8 P.F. Michelson 9 N Mirabal 13 W. Mitthumsiri 35 T. Mizuno 36 M.E. Monzani 9 A. Morselli 37 I. V. Moskalenko 10 M. Negro 12 E. Nuss 11 T. Ohsugi 36 N. Omodei 10 E. Orlando 9 J. F. Ormes 38 D. Paneque 39 M. Persic 40 M. Pesce-Rollins 4 F. Piron 11 T.A. Porter 9 G. Principe 23 S. Rainò 8 R. Rando 21 M. Razzano 41 O. Reimer 42 M. Sánchez Conde 43 C. Sgró 14 D. Simone 8 E. J. Siskind 44 F Spada 4 G. Spandre 4 P. Spinelli 8 K Tanaka 36 L. Tibaldo 45 D.F Torres 46 E. Troja 13 Y Uchiyama 47 J.C. Wang 31 K.S. Wood 48 M. Wood 9 G. Zaharijas 6 M. Zhou 27
Abstract : The Fermi Large Area Telescope (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, Local Group galaxy M31 was detected as a marginally extended gamma-ray source, while only an upper limit has been derived for the other nearby galaxy M33. We revisited the gamma-ray emission in the direction of M31 and M33 using more than 7 yr of LAT Pass 8 data in the energy range – 0.1 100 GeV, presenting detailed morphological and spectral analyses. M33 remains undetected, and we computed an upper limit of´-of´of´-2.0 10 erg cm s 12 2 1 on the – 0.1 100 GeV energy flux (95% confidence level). This revised upper limit remains consistent with the observed correlation between gamma-ray luminosity and star formation rate tracers and implies an average CR density in M33 that is at most half of that of the MW. M31 is detected with a significance of nearly s 10. Its spectrum is consistent with a power law with photon index G =  + 2.4 0.1 stat syst and a – 0.1 100 GeV energy flux of ()  ´ +-5.6 0.6 10 erg cm s stat syst 12 2 1. M31 is detected to be extended with a s 4 significance. The spatial distribution of the emission is consistent with a uniform-brightness disk with a radius of 0°. 4 and no offset from the center of the galaxy, 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 disk of the galaxy or extend far from it. The gamma-ray signal is not correlated with regions rich in gas or star formation activity, which suggests 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.
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M Ackermann, M Ajello, A. Albert, L. Baldini, Jean Ballet, et al.. Observations of M31 and M33 with the Fermi Large Area Telescope: A Galactic Center Excess in Andromeda?. The Astrophysical Journal, American Astronomical Society, 2017, 836, pp.208 - 208. ⟨10.3847/1538-4357/aa5c3d⟩. ⟨insu-01522758⟩

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