Letter to the Editor

Probing the molecular interstellar medium of M82 with Herschel-SPIRE spectroscopy^*

¹ CEA, Laboratoire AIM, Irfu/SAp, Orme des Merisiers, 91191 Gif-sur-Yvette, France e-mail: pasquale.panuzzo@cea.fr
² Dept. of Astrophysical & Planetary Sciences, CASA CB-389, University of Colorado, Boulder, CO 80309, USA
³ School of Physics & Astronomy, Cardiff University, Queens Buildings The Parade, Cardiff CF24 3AA, UK
⁴ ESA Astrophysics Missions Division, ESTEC, PO Box 299, 2200 AG Noordwijk, The Netherlands
⁵ Dept. of Physics & Astronomy, McMaster University, Hamilton, Ontario, L8S 4M1, Canada
⁶ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9, 9000 Gent, Belgium
⁷ Dept. of Physics & Astronomy, University College London, Gower Street, London WC1E 6BT, UK
⁸ Astrophysics Group, Imperial College, Blackett Laboratory, Prince Consort Road, London SW7 2AZ, UK
⁹ JPL, Pasadena, CA 91109, United States; Dept. of Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
¹⁰ Laboratoire d'Astrophysique de Marseille, UMR6110 CNRS, 38 rue F. Joliot-Curie, 13388 Marseille, France
¹¹ Instituto de Astrofísica de Canarias (IAC) and Departamento de Astrofísica, Universidad de La Laguna (ULL), La Laguna, Tenerife, Spain
¹² Institut d'Astrophysique de Paris, UMR7095 CNRS, Université Pierre & Marie Curie, 98 bis Boulevard Arago, 75014 Paris, France
¹³ Dept. of Physics & Astronomy, University of California, Irvine, CA 92697, USA
¹⁴ Observational Cosmology Lab, Code 665, NASA Goddard Space Flight Center Greenbelt, MD 20771, USA
¹⁵ Blue Sky Spectroscopy, Alberta, T1J 1B1, Canada
¹⁶ Dept. of Physics and Astronomy, University of Sussex, Brighton, BN1 9QH, UK
¹⁷ Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK
¹⁸ Space Science and Technology Department, Rutherford Appleton Laboratory, Oxfordshire, OX11 0QX, UK
¹⁹ Institute for Space Imaging Science, University of Lethbridge, Lethbridge, Alberta, T1K 3M4, Canada
²⁰ School of Physics & Astronomy, University of Nottingham, University Park, Nottingham NG7 2RD, UK
²¹ Istituto di Fisica dello Spazio Interplanetario, INAF, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
²² IPAC, California Institute of Technology, Mail Code 100-22, 770 South Wilson Av, Pasadena, CA 91125, USA
²³ Centre for Astrophysics Research, Science & Technology Research Centre, University of Hertfordshire, College Lane, Herts AL10 9AB, UK
²⁴ University of Padova, Dept. of Astronomy, Vicolo Osservatorio 3, 35122 Padova, Italy
²⁵ Observatoire Astronomique de Strasbourg, UMR 7550 Université de Strasbourg - CNRS, 11, rue de l'Université, 67000 Strasbourg, France
²⁶ UK Astronomy Technology Center, Royal Observatory Edinburgh, Edinburgh, EH9 3HJ, UK
²⁷ Institut für Astronomie, Universität Wien, Türkenschanzstr. 17, 1180 Wien, Austria

Received: 30 March 2010
Accepted: 28 April 2010

Abstract

We present the observations of the starburst galaxy M82 taken with the Herschel SPIRE Fourier-transform spectrometer. The spectrum (194–671 μm) shows a prominent CO rotational ladder from J = 4–3 to 13–12 emitted by the central region of M82. The fundamental properties of the gas are well constrained by the high J lines observed for the first time. Radiative transfer modeling of these high-S/N ¹²CO and ¹³CO lines strongly indicates a very warm molecular gas component at ~500 K and pressure of ~3×10⁶ K cm^-3, in good agreement with the H₂ rotational lines measurements from Spitzer and ISO. We suggest that this warm gas is heated by dissipation of turbulence in the interstellar medium (ISM) rather than X-rays or UV flux from the straburst. This paper illustrates the promise of the SPIRE FTS for the study of the ISM of nearby galaxies.