Letter to the Editor

Excitation of the molecular gas in the nuclear region of M 82

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands e-mail: loenen@strw.leidenuniv.nl
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ Observatorio Astronomico Nacional, Apdo. 1143, 28800 Alcalá de Henares (Madrid), Spain
⁴ Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA
⁵ Instituto Radioastronomía Milimétrica (IRAM), Av. Divina Pastora 7, Nucleo Central, 18012 Granada, Spain
⁶ NASA Science Center, California Institute of Technology, M.S. 100–22, Pasadena, CA 91125, USA
⁷ LAM, CAB-CSIC/INTA, Ctra de Torrejón a Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁸ I. Physikalisches Institut der Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
⁹ N. Copernicus Astronomical Center (NCAC), Rabianska 8, 87100 Torun, Poland
¹⁰ Jet Propulsion Laboratory, M/S 169-506, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
¹¹ Centre d'Étude Spatiale des Rayonnements, Université de Toulouse [UPS], 31062 Toulouse Cedex 9, France
¹² CNRS/INSU, UMR 5187, 9 avenue du Colonel Roche, 31028 Toulouse Cedex 4, France
¹³ Laboratoire d'Études du Rayonnement et de la Matière en Astrophysique, UMR 8112 CNRS/INSU, OP, ENS, UPMC, UCP, Paris, France and LERMA, Observatoire de Paris, 61 avenue de l'Observatoire, 75014 Paris, France
¹⁴ SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands
¹⁵ Osservatorio Astrofisico di Arcetri-INAF- Largo E. Fermi 5, 50100 Florence, Italy
¹⁶ Infrared Processing and Analysis Center, California Institute of Technology, MS 100-22, Pasadena, CA 91125, USA
¹⁷ California Institute of Technology, Cahill Center for Astronomy and Astrophysics 301-17, Pasadena, CA 91125, USA

Received: 31 May 2010
Accepted: 25 June 2010

Abstract

We present high-resolution HIFI spectroscopy of the nucleus of the archetypical starburst galaxy M 82. Six ¹²CO lines, 2 ¹³CO lines and 4 fine-structure lines have been detected. Besides showing the effects of the overall velocity structure of the nuclear region, the line profiles also indicate the presence of multiple components with different optical depths, temperatures, and densities in the observing beam. The data have been interpreted using a grid of PDR models. It is found that the majority of the molecular gas is in low density (n = 10^3.5 cm^-3) clouds, with column densities of N_H = 10^21.5 cm^-2 and a relatively low UV radiation field (G₀ = 10²). The remaining gas is predominantly found in clouds with higher densities (n = 10⁵ cm^-3) and radiation fields (G₀ = 10^2.75), but somewhat lower column densities (N_H = 10^21.2 cm^-2). The highest J CO lines are dominated by a small (1% relative surface filling) component, with an even higher density (n = 10⁶ cm^-3) and UV field (G₀ = 10^3.25). These results show the strength of multi-component modelling for interpretating the integrated properties of galaxies.