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

Ackermann M, Ajello M, Albert A, Baldini L, Ballet J, Barbiellini G, Bastieri D, Bellazzini R, Bissaldi E, Bloom ED, Bonino R, Bottacini E, Brandt TJ, Bregeon J, Bruel P, Buehler R, Cameron RA, Caputo R, Caragiulo M, Caraveo PA, Cavazzuti E, Cecchi C, Charles E, Chekhtman A, Chiaro G, Ciprini S, Costanza F, Cutini S, D'Ammando F, De Palma F, Desiante R, Digel SW, Di Lalla N, Di Mauro M, Di Venere L, Favuzzi C, Funk S, Fusco P, Gargano F, Giglietto N, Giordano F, Giroletti M, Glanzman T, Green D, Grenier IA, Guillemot L, Guiriec S, Hayashi K, Hou X, Johannesson G, Kamae T, Knodlseder J, Kong AKH, Kuss M, La Mura G, Larsson S, Latronico L, Li J, Longo F, Loparco F, Lubrano P, Maldera S, Malyshev D, Manfreda A, Martin P, Mazziotta MN, Michelson PF, Mirabal N, Mitthumsiri W, Mizuno T, Monzani ME, Morselli A, Moskalenko IV, Negro M, Nuss E, Ohsugi T, Omodei N, Orlando E, Ormes JF, Paneque D, Persic M, Pesce-Rollins M, Piron F, Porter TA, Principe G, Raino S, Rando R, Razzano M, Reimer O, Sanchez-Conde M, Sgro C, Simone D, Siskind EJ, Spada F, Spandre G, Spinelli P, Tanaka K, Tibaldo L, Torres DF, Troja E, Uchiyama Y, Wang JC, Wood KS, Wood M, Zaharijas G, Zhou M (2017)

Publication Type: Journal article

Publication year: 2017

Journal

Book Volume: 836

Journal Issue: 2

DOI: 10.3847/1538-4357/aa5c3d

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 2.0 x 10(-12) erg cm(-2) s(-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 10 sigma. Its spectrum is consistent with a power law with photon index Gamma = 2.4 +/- 0.1(stat) (vertical bar) (syst) and a 0.1-100 GeV energy flux of (5.6 +/- 0.6(stat vertical bar syst)) x 10(-12) erg cm(-1) s(-1). M31 is detected to be extended with a 4 sigma significance. The spatial distribution of the emission is consistent with a uniform-brightness disk with a radius of 0 degrees.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.

Authors with CRIS profile

Involved external institutions

National Aeronautics and Space Administration (NASA) United States (USA) (US) Nagoya University / 名古屋大学 Japan (JP) University of Pisa / Università di Pisa (UniPi) Italy (IT) National Institute for Nuclear Physics / Istituto Nazionale di Fisica Nucleare (INFN) Italy (IT) Chinese Academy of Sciences (CAS) / 中国科学院 China (CN) University of Paris 7 - Denis Diderot / Université Paris VII Denis Diderot France (FR) National Tsing Hua University (NTHU) Taiwan (TW) Mahidol University / มหาวิทยาลัยมหิดล Thailand (TH) University of Orléans / Université d'Orléans France (FR) University of Iceland (UI) / Háskóli Íslands Iceland (IS) Royal Institute of Technology / Kungliga Tekniska Högskolan (KTH) Sweden (SE) University of Denver United States (USA) (US) University of Tokyo Japan (JP) Max-Planck-Institut für Physik (Werner-Heisenberg-Institut, MPP) / Max Planck Institute for Physics Germany (DE) Stanford University United States (USA) (US) University of Montpellier / Université Montpellier France (FR) École Polytechnique - Université Paris-Saclay France (FR) NYCB Real-Time Computing, Inc. United States (USA) (US) Hiroshima University / 広島大学 Japan (JP) Praxis, Inc. United States (USA) (US) National Center for Scientific Research / Centre national de la recherche scientifique (CNRS) France (FR) Deutsches Elektronen-Synchrotron DESY Germany (DE) National Institute for Astrophysics / Istituto Nazionale Astrofisica (INAF) Italy (IT) University of Padua / Universita degli Studi di Padova Italy (IT) Italian Space Agency / Agenzia Spaziale Italiana (ASI) Italy (IT) Clemson University United States (USA) (US) Rikkyo University / 立教大学 Japan (JP) Los Alamos National Laboratory United States (USA) (US) Spanish National Research Council / Consejo Superior de Investigaciones Científicas (CSIC) Spain (ES) University of California Santa Cruz United States (USA) (US) George Mason University United States (USA) (US) Max-Planck-Institut für Kernphysik (MPIK) / Max Planck Institute for Nuclear Physics Germany (DE) AlbaNova University Centre / Kungliga Tekniska Högskolan AlbaNova Universitetscentrum Sweden (SE)

How to cite

APA:

Ackermann, M., Ajello, M., Albert, A., Baldini, L., Ballet, J., Barbiellini, G.,... Zhou, M. (2017). Observations of M31 and M33 with the Fermi Large Area Telescope: A Galactic Center Excess in Andromeda? Astrophysical Journal, 836(2). https://doi.org/10.3847/1538-4357/aa5c3d

MLA:

Ackermann, Markus, et al. "Observations of M31 and M33 with the Fermi Large Area Telescope: A Galactic Center Excess in Andromeda?" Astrophysical Journal 836.2 (2017).

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