The cosmic-ray and gas content of the Cygnus region as measured in γ-rays by the Fermi Large Area Telescope

¹ 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
² Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, 56127 Pisa, Italy
³ Laboratoire AIM, CEA-IRFU/CNRS/Université Paris Diderot, Service d’Astrophysique, CEA Saclay, 91191 Gif-sur-Yvette, France
e-mail: isabelle.grenier@cea.fr
⁴ Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, 34127 Trieste, Italy
⁵ Dipartimento di Fisica, Università di Trieste, 34127 Trieste, Italy
⁶ Istituto Nazionale di Fisica Nucleare, Sezione di Padova, 35131 Padova, Italy
e-mail: luigi.tibaldo@pd.infn.it
⁷ Dipartimento di Fisica “G. Galilei”, Università di Padova, 35131 Padova, Italy
⁸ INAF-Istituto di Astrofisica Spaziale e Fisica Cosmica, 20133 Milano, Italy
⁹ Istituto Nazionale di Fisica Nucleare, Sezione di Perugia, 06123 Perugia, Italy
¹⁰ Dipartimento di Fisica, Università degli Studi di Perugia, 06123 Perugia, Italy
¹¹ Dipartimento di Fisica “M. Merlin” dell’Università e del Politecnico di Bari, 70126 Bari, Italy
¹² Istituto Nazionale di Fisica Nucleare, Sezione di Bari, 70126 Bari, Italy
¹³ Laboratoire Leprince-Ringuet, École polytechnique, CNRS/IN2P3, Palaiseau, France
¹⁴ Institut de Ciències de l’Espai (IEEE-CSIC), Campus UAB, 08193 Barcelona, Spain
¹⁵ Artep Inc., 2922 Excelsior Springs Court, Ellicott City, MD 21042, resident at Naval Research Laboratory, Washington, DC 20375, USA
¹⁶ ASI Science Data Center, 00044 Frascati, Roma, Italy
¹⁷ Laboratoire Univers et Particules de Montpellier, Université Montpellier 2, CNRS/IN2P3, Montpellier, France
¹⁸ Dipartimento di Fisica, Università di Udine and Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, Gruppo Collegato di Udine, 33100 Udine, Italy
¹⁹ Space Science Division, Naval Research Laboratory, Washington, DC 20375-5352, USA
²⁰ Université Bordeaux 1, CNRS/IN2p3, Centre d’Études Nucléaires de Bordeaux Gradignan, 33175 Gradignan, France
²¹ Department of Physical Sciences, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan
²² INAF Istituto di Radioastronomia, 40129 Bologna, Italy
²³ Current address: Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
²⁴ Center for Space Plasma and Aeronomic Research (CSPAR), University of Alabama in Huntsville, Huntsville, AL 35899, USA
²⁵ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
²⁶ Science Institute, University of Iceland, IS-107 Reykjavik, Iceland
²⁷ College of Science, Ibaraki University, 2-1-1, Bunkyo, Mito 310-8512, Japan
²⁸ Research Institute for Science and Engineering, Waseda University, 3-4-1, Okubo, Shinjuku, Tokyo 169-8555, Japan
²⁹ CNRS, IRAP, 31028 Toulouse Cedex 4, France
³⁰ Université de Toulouse, UPS-OMP, IRAP, Toulouse, France
³¹ Yukawa Institute for Theoretical Physics, Kyoto University, Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
³² Max-Planck Institut für extraterrestrische Physik, 85748 Garching, Germany
³³ Department of Physics and Department of Astronomy, University of Maryland, College Park, MD 20742, USA
³⁴ Istituto Nazionale di Fisica Nucleare, Sezione di Roma “Tor Vergata”, 00133 Roma, Italy
³⁵ Department of Physics, Boise State University, Boise, ID 83725, USA
³⁶ Hiroshima Astrophysical Science Center, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan
³⁷ Institute of Space and Astronautical Science, JAXA, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan
³⁸ Department of Physics and Astronomy, University of Denver, Denver, CO 80208, USA
³⁹ Max-Planck-Institut für Physik, 80805 München, Germany
⁴⁰ Center for Earth Observing and Space Research, College of Science, George Mason University, Fairfax, VA 22030, resident at Naval Research Laboratory, Washington, DC 20375, USA
⁴¹ Institut für Astro- und Teilchenphysik and Institut fürTheoretische Physik, Leopold-Franzens-Universität Innsbruck, 6020 Innsbruck, Austria
⁴² 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
⁴³ NYCB Real-Time Computing Inc., Lattingtown, NY 11560-1025, USA
⁴⁴ Department of Physics, Center for Cosmology and Astro-Particle Physics, The Ohio State University, Columbus, OH 43210, USA
⁴⁵ Partially supported by the International Doctorate on Astroparticle Physics (IDAPP) program  
⁴⁶ Institució Catalana de Recerca i Estudis Avançats (ICREA), Barcelona, Spain
⁴⁷ Consorzio Interuniversitario per la Fisica Spaziale (CIFS), 10133 Torino, Italy
⁴⁸ INTEGRAL Science Data Centre, 1290 Versoix, Switzerland
⁴⁹ NASA Postdoctoral Program Fellow, USA
⁵⁰ Dipartimento di Fisica, Università di Roma “Tor Vergata”, 00133 Roma, Italy
⁵¹ Department of Physics, Stockholm University, AlbaNova, 106 91 Stockholm, Sweden
⁵² The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova, 106 91 Stockholm, Sweden
⁵³ Laboratoire d’Astrophysique de Bordeaux, Université de Bordeaux, CNRS/INSU, Floirac Cedex, France

Received: 22 June 2011
Accepted: 29 October 2011

Abstract

Context. The Cygnus region hosts a giant molecular-cloud complex that actively forms massive stars. Interactions of cosmic rays with interstellar gas and radiation fields make it shine at γ-ray energies. Several γ-ray pulsars and other energetic sources are seen in this direction.

Aims. In this paper we analyze the γ-ray emission measured by the Fermi Large Area Telescope (LAT) in the energy range from 100 MeV to 100 GeV in order to probe the gas and cosmic-ray content on the scale of the whole Cygnus complex. The γ-ray emission on the scale of the central massive stellar clusters and from individual sources is addressed elsewhere.

Methods. The signal from bright pulsars is greatly reduced by selecting photons in their off-pulse phase intervals. We compare the diffuse γ-ray emission with interstellar gas maps derived from radio/mm-wave lines and visual extinction data. A general model of the region, including other pulsars and γ-ray sources, is sought.

Results. The integral H i emissivity above 100 MeV averaged over the whole Cygnus complex amounts to [2.06 ± 0.11 (stat.) _-0.84^+0.15(syst.)] × 10^-26 photons s^-1 sr^-1 H-atom^-1, where the systematic error is dominated by the uncertainty on the H i opacity to calculate its column densities. The integral emissivity and its spectral energy distribution are both consistent within the systematics with LAT measurements in the interstellar space near the solar system. The average X_CO = N(H₂)/W_CO ratio is found to be [1.68 ± 0.05 (stat.) _-0.10^+0.87(HI opacity)] × 10²⁰ molecules cm^-2 (K km s^-1)^-1, consistent with other LAT measurements in the Local Arm. We detect significant γ-ray emission from dark neutral gas for a mass corresponding to  ~40% of what is traced by CO. The total interstellar mass in the Cygnus complex inferred from its γ-ray emission amounts to 8_-1⁺⁵ × 10⁶ M_⊙ at a distance of 1.4 kpc.

Conclusions. Despite the conspicuous star formation activity and high masses of the interstellar clouds, the cosmic-ray population in the Cygnus complex averaged over a few hundred parsecs is similar to that of the local interstellar space.