A&A 492, 675-684 (2008)
On the chemistry and distribution of HOC in M 82
More evidence for extensive PDRsA. Fuente1, S. García-Burillo1, A. Usero1, 2, M. Gerin3, R. Neri4, A. Faure5, J. Le Bourlot6, M. González-García6, J. R. Rizzo7, T. Alonso-Albi1, and J. Tennyson8
1 Observatorio Astronómico Nacional (OAN), Apdo. 112, 28800 Alcalá de Henares, Madrid, Spain
2 Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
3 Laboratoire d'Étude du Rayonnement et de la Matière, UMR 8112, CNRS, École Normale Supérieure et Observatoire de Paris, 24 rue Lhomond, 75231 Paris Cedex 05, France
4 Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 St Martin d'Hères, France
5 Laboratoire d'Astrophysique Observatoire de Grenoble, BP 53, 38041 Grenoble Cedex 9, France
6 LUTH, Observatoire de Paris and Université Paris, 7 place Jansen, 92190 Meudon, France
7 Laboratorio de Astrofísica Espacial y Física Fundamental, Apdo 78, 28691 Villanueva de la Cañada, Madrid, Spain
8 Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
Received 11 July 2008 / Accepted 3 October 2008
Context. The molecular gas composition in the inner 1 kpc disk of the starburst galaxy M 82 resembles that of Galactic Photon Dominated Regions (PDRs). In particular, large abundances of the reactive ions HOC+ and CO+ have been measured in the nucleus of this galaxy. Two explanations have been proposed for such high abundances: the influence of intense UV fields from massive stars, or a significant role of X-Rays.
Aims. Our aim is to investigate the origin of the high abundances of reactive ions in M 82.
Methods. We have completed our previous 30 m HOC+ observations with the higher excitation HCO+ and HOC+ and rotational lines. In addition, we have obtained with the IRAM Plateau de Bure Interferometer (PdBI) a 4'' resolution map of the HOC+ emission in M 82, the first ever obtained in a Galactic or extragalactic source.
Results. Our HOC+ interferometric image shows that the emission of the HOC+ line is mainly restricted to the nuclear disk, with the maxima towards the E and W molecular peaks. In addition, line excitation calculations imply that the HOC+ emission arises in dense gas ( 104 cm-3). Therefore, the HOC+ emission is arising in the dense PDRs embedded in the M 82 nuclear disk, rather than in the intercloud phase and/or wind.
Conclusions. We have improved our previous chemical model of M 82 by (i) using the new version of the Meudon PDR code; (ii) updating the chemical network; and (iii) considering two different types of clouds (with different thickness) irradiated by the intense interstellar UV field (G0 = 104 in units of the Habing field) prevailing in the nucleus of M 82. Most molecular observations (HCO+, HOC+, CO+, CN, HCN, H3O+) are well explained assuming that ~87% of the mass of the molecular gas is forming small clouds (Av = 5 mag) while only ~13% of the mass is in large molecular clouds (Av = 50 mag). Such a small number of large molecular clouds suggests that M 82 is an old starburst, where star formation has almost exhausted the molecular gas reservoir.
Key words: galaxies: individual: M 82 -- galaxies: nuclei -- galaxies: starburst -- ISM: molecules -- ISM: abundances -- radio lines: galaxies
© ESO 2008