2007A&A...473..771C

We investigate the molecular gas content and the excitation and fragmentation properties in the central region of the spiral galaxy Messier 81 in both the ¹²CO(1-0) and ¹²CO(2-1) transitions. We have recently observed the two transitions of CO in the M 81 center with A, B, and HERA receivers of the IRAM 30-m telescope. We find no CO emission in the inner ∼300pc and a weak molecular gas clump structure at a distance of around 460pc from the nucleus. Observations of the first two CO transitions allowed us to compute the line ratio, and the average I₂₁/I₁₀ ratio is 0.68 for the M 81 center. This low value, atypical both of the galactic nuclei of spiral galaxies and of interacting systems, is probably associated to diffuse gas with molecular hydrogen density that is not high enough to excite the CO molecules. After analyzing the clumping properties of the molecular gas in detail, we identify very massive giant molecular associations (GMAs) in CO(2-1) emission with masses of ∼10⁵M_☉ and diameters of ∼250pc. The deduced N(H₂)/I_CO ratio for the individually resolved GMAs, assumed to be virialized, is a factor of ∼15 higher than the standard Galactic value, showing - as suspected - that the X ratio departs significantly from the mean for galaxies with an unusual physics of the molecular gas.