On the chemistry and distribution of HOC in M 82

More evidence for extensive PDRs

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

Abstract

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 (A_v = 5 mag) while only ~13% of the mass is in large molecular clouds (A_v = 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.