2008A&A...492..675F

The molecular gas composition in the inner 1kpc 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. Our aim is to investigate the origin of the high abundances of reactive ions in M 82. We have completed our previous 30m HOC⁺ J=1->0 observations with the higher excitation HCO⁺ and HOC⁺ J=4->3 and 3->2 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. Our HOC⁺ interferometric image shows that the emission of the HOC⁺ 1->0 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≥10⁴cm^–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. 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=5mag) while only ∼13% of the mass is in large molecular clouds (A_v=50mag). 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.