Letter to the Editor

Fermi Large Area Telescope observations of Local Group galaxies: detection of M 31 and search for M 33^⋆

¹ Space Science DivisionNaval Research Laboratory, Washington, DC 20375, USA
² National Research Council Research Associate, National Academy of Sciences, Washington, DC 20001, USA
³ W. W. Hansen Experimental Physics Laboratory, Kavli Institute for Particle Astrophysics and Cosmology, Department of Physics and SLAC National Accelerator Laboratory, Stanford University, Stanford, CA 94305, USA
e-mail: bechtol@stanford.edu
⁴ Santa Cruz Institute for Particle Physics, Department of Physics and Department of Astronomy and Astrophysics, University of California at Santa Cruz, Santa Cruz, CA 95064, USA
⁵ Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, 56127 Pisa, Italy
⁶ Laboratoire AIM, CEA-IRFU/CNRS/Université Paris Diderot, Service d’Astrophysique, CEA Saclay, 91191 Gif-sur-Yvette, France
⁷ Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, 34127 Trieste, Italy
⁸ Dipartimento di Fisica, Università di Trieste, 34127 Trieste, Italy
⁹ Istituto Nazionale di Fisica Nucleare, Sezione di Padova, 35131 Padova, Italy
¹⁰ Dipartimento di Fisica “G. Galilei”, Università di Padova, 35131 Padova, Italy
¹¹ Istituto Nazionale di Fisica Nucleare, Sezione di Perugia, 06123 Perugia, Italy
¹² Dipartimento di Fisica, Università degli Studi di Perugia, 06123 Perugia, Italy
¹³ Centre d’Étude Spatiale des Rayonnements, CNRS/UPS, BP 44346, 31028 Toulouse Cedex 4, France
e-mail: knodlseder@cesr.fr; jean@cesr.fr
¹⁴ Department of Physics, Center for Cosmology and Astro-Particle Physics, The Ohio State University, Columbus, OH 43210, USA
¹⁵ Dipartimento di Fisica “M. Merlin” dell’Università e del Politecnico di Bari, 70126 Bari, Italy
¹⁶ Istituto Nazionale di Fisica Nucleare, Sezione di Bari, 70126 Bari, Italy
¹⁷ Laboratoire Leprince-Ringuet, École polytechnique, CNRS/IN2P3, Palaiseau, France
¹⁸ Department of Physics, University of Washington, Seattle, WA 98195-1560, USA
¹⁹ Institut de Ciencies de l’Espai (IEEC-CSIC), Campus UAB, 08193 Barcelona, Spain
²⁰ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
²¹ University College Dublin, Belfield, Dublin 4, Ireland
²² INAF-Istituto di Astrofisica Spaziale e Fisica Cosmica, 20133 Milano, Italy
²³ Center for Research and Exploration in Space Science and Technology (CRESST) and NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
²⁴ Department of Physics and Center for Space Sciences and Technology, University of Maryland Baltimore County, Baltimore, MD 21250, USA
²⁵ George Mason University, Fairfax, VA 22030, USA
²⁶ Laboratoire de Physique Théorique et Astroparticules, Université Montpellier 2, CNRS/IN2P3, Montpellier, France
²⁷ Department of Physics, Stockholm University, AlbaNova, 10691 Stockholm, Sweden
²⁸ The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova, 10691 Stockholm, Sweden
²⁹ Royal Swedish Academy ofSciences Research Fellow, funded by a grant from the K. A. Wallenberg Foundation, Sweden
³⁰ Dipartimento di Fisica, Università di Udine and Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, Gruppo Collegato di Udine, 33100 Udine, Italy
³¹ Osservatorio Astronomico di Trieste, Istituto Nazionale di Astrofisica, 34143 Trieste, Italy
³² Department of Physical Sciences, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan
³³ INAF Istituto di Radioastronomia, 40129 Bologna, Italy
³⁴ Université Bordeaux 1, CNRS/IN2p3, Centre d’Études Nucléaires de Bordeaux Gradignan, 33175 Gradignan, France
³⁵ Center for Space Plasma and Aeronomic Research (CSPAR), University of Alabama in Huntsville, Huntsville, AL 35899, USA
³⁶ Science Institute, University of Iceland, 107 Reykjavik, Iceland
³⁷ Department of Physics and Department of Astronomy, University of Maryland, College Park, MD 20742, USA
³⁸ Research Institute for Science and Engineering, Waseda University, 3-4-1, Okubo, Shinjuku, Tokyo, 169-8555 Japan
³⁹ Max-Planck Institut für extraterrestrische Physik, 85748 Garching, Germany
e-mail: martinp@mpe.mpg.de
⁴⁰ Istituto Nazionale di Fisica Nucleare, Sezione di Roma “Tor Vergata”, 00133 Roma, Italy
⁴¹ Department of Physics and Astronomy, University of Denver, Denver, CO 80208, USA
⁴² Hiroshima Astrophysical Science Center, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan
⁴³ Institute of Space and Astronautical Science, JAXA, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan
⁴⁴ Institut für Astro- und Teilchenphysik and Institut für Theoretische Physik, Leopold-Franzens-Universität Innsbruck, 6020 Innsbruck, Austria
⁴⁵ NYCB Real-Time Computing Inc., Lattingtown, NY 11560-1025, USA
⁴⁶ Department of Chemistry and Physics, Purdue University Calumet, Hammond, IN 46323-2094, USA
⁴⁸ Institució Catalana de Recerca i Estudis Avançats (ICREA), Barcelona, Spain
⁴⁹ Consorzio Interuniversitario per la Fisica Spaziale (CIFS), 10133 Torino, Italy
⁵⁰ INTEGRAL Science Data Centre, CH-1290 Versoix, Switzerland
⁵¹ Dipartimento di Fisica, Università di Roma “Tor Vergata”, 00133 Roma, Italy

Received: 15 September 2010
Accepted: 19 October 2010

Abstract

Context. Cosmic rays (CRs) can be studied through the galaxy-wide gamma-ray emission that they generate when propagating in the interstellar medium. The comparison of the diffuse signals from different systems may inform us about the key parameters in CR acceleration and transport.

Aims. We aim to determine and compare the properties of the cosmic-ray-induced gamma-ray emission of several Local Group galaxies.

Methods. We use 2 years of nearly continuous sky-survey observations obtained with the Large Area Telescope aboard the Fermi Gamma-ray Space Telescope to search for gamma-ray emission from M 31 and M 33. We compare the results with those for the Large Magellanic Cloud, the Small Magellanic Cloud, the Milky Way, and the starburst galaxies M 82 and NGC 253.

Results. We detect a gamma-ray signal at 5σ significance in the energy range 200 MeV–20 GeV that is consistent with originating from M 31. The integral photon flux above 100 MeV amounts to (9.1 ± 1.9_stat ± 1.0_sys) × 10^-9 ph cm^-2 s^-1. We find no evidence for emission from M 33 and derive an upper limit on the photon flux  >100 MeV of 5.1 × 10^-9 ph cm^-2 s^-1 (2σ). Comparing these results to the properties of other Local Group galaxies, we find indications of a correlation between star formation rate and gamma-ray luminosity that also holds for the starburst galaxies.

Conclusions. The gamma-ray luminosity of M 31 is about half that of the Milky Way, which implies that the ratio between the average CR densities in M 31 and the Milky Way amounts to ξ = 0.35 ± 0.25. The observed correlation between gamma-ray luminosity and star formation rate suggests that the flux of M 33 is not far below the current upper limit from the LAT observations.