2000A&A...358..433M

¹²CO J=1-0, 2-1, 4-3, 7-6, and ¹³CO 1-0, 2-1, and 3-2 line emission was mapped with angular resolutions of 13''-22'' toward the nuclear region of the archetypical starburst galaxy M82. There are two hotspots on either side of the dynamical center, with the south-western lobe being slightly more prominent. Lobe spacings are not identical for all transitions: For the submillimeter CO lines, the spacing is ∼15''; for the millimeter lines (CO J=2-1 and 1-0) the spacing is ∼26'', indicating the presence of a `low' and a `high' CO excitation component. A Large Velocity Gradient (LVG) excitation analysis of the submillimeter lines leads to inconsistencies, since area and volume filling factors are almost the same, resulting in cloud sizes along the lines-of-sight that match the entire size of the M82 starburst region. Nevertheless, LVG column densities agree with estimates derived from the dust emission in the far infrared and at submillimeter wavelengths. 22'' beam averaged total column densities are N(CO)∼5x10¹⁸ and N(H₂)∼10²³cm^–2; the total molecular mass is a few 10⁸M_☉. Accounting for high UV fluxes and variations in kinetic temperature and assuming that the observed emission arises from photon dominated regions (PDRs) resolves the problems related to an LVG treatment of the radiative transfer. Spatial densities are as in the LVG case (n(H₂)∼10^3.7cm^–3 and ∼10³cm^–3 for the high and low excitation component, respectively), but ¹²CO/¹³CO intensity ratios >10 indicate that the bulk of the CO emission arises in UV-illuminated diffuse cloud fragments of small column density (N(H₂)∼5x10²⁰cm^–2/km/s) and sub-parsec cloud sizes with area filling factors ≫1. Thus CO arises from quite a different gas component than the classical high density tracers (e.g. CS, HCN) that trace star formation rates more accurately. The dominance of such a diffuse molecular interclump medium also explains observed high [Ci]/CO line intensity ratios. PDR models do not allow a determination of the relative abundances of ¹²CO to ¹³CO. Ignoring magnetic fields, the CO emitting gas appears to be close to the density limit for tidal disruption. Neither changes in the ¹²C/¹³C abundance ratio nor variations of the incident far-UV flux provide good fits to the data for simulations of larger clouds. A warm diffuse ISM not only dominates the CO emission in the starburst region of M82 but is also ubiquitous in the central region of our Galaxy, where tidal stress, cloud-cloud collisions, shocks, high gas pressure, and high stellar densities may all contribute to the formation of a highly fragmented molecular debris. ¹²CO, ¹²CO/¹³CO, and [C i]/CO line intensity ratios in NGC 253 (and NGC 4945) suggest that the CO emission from the centers of these galaxies arises in a physical environment that is similar to that in M82. Starburst galaxies at large distances (z∼2.2-4.7) show ¹²CO line intensity ratios that are consistent with those observed in M82. PDR models should be applicable to all these sources. ¹²CO/¹³CO line intensity ratios ≫10, sometimes observed in nearby ultraluminous mergers, require the presence of a particularly diffuse, extended molecular medium. Here [CI]/CO abundance ratios should be as large or even larger than in M 82 and NGC 253.