Volume 492, Number 3, December IV 2008
|Page(s)||675 - 684|
|Published online||06 November 2008|
On the chemistry and distribution of HOC in M 82
More evidence for extensive PDRs
Observatorio Astronómico Nacional (OAN), Apdo. 112, 28800 Alcalá de Henares, Madrid, Spain e-mail: email@example.com
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
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
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.