EDP Sciences
Free Access
Volume 492, Number 3, December IV 2008
Page(s) 675 - 684
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361:200810571
Published online 06 November 2008

A&A 492, 675-684 (2008)
DOI: 10.1051/0004-6361:200810571

On the chemistry and distribution of HOC $\mathsf{^+}$ in M 82

More evidence for extensive PDRs
A. 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
    e-mail: a.fuente@oan.es
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+ $J=1\rightarrow0$ observations with the higher excitation HCO+ and HOC+  $J=4\rightarrow3$ and $3\rightarrow2$ 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+  $1\rightarrow0$ 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 ($n\ge$ 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

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