Fuente, A.; García-Burillo, S.; Usero, A.; Gerin, M.; Neri, R.; Faure, A.; Le Bourlot, J.; ... Tennyson, J.; + view all Fuente, A.; García-Burillo, S.; Usero, A.; Gerin, M.; Neri, R.; Faure, A.; Le Bourlot, J.; González-García, M.; Rizzo, J.R.; Alonso-Albi, T.; Tennyson, J.; - view fewer (2008) On the chemistry and distribution of HOC+ in M 82: more evidence for extensive PDRs. Astronomy and Astrophysics , 492 (3) pp. 675-684. 10.1051/0004-6361:200810571.

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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{^+} 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\geq 10{^4} 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 (G{_0} = 10{^4} in units of the Habing field) prevailing in the nucleus of M 82. Most molecular observations (HCO{^+}, HOC{^+}, CO{^+}, CN, HCN, H{_3}O{^+}) 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.

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