1998A&A...333...92B

We report about high spatial resolution observations made with the IRAM Plateau de Bure interferometer of the ¹²CO(J=1->0) emission from a 1.1x1.1kpc plane-of-sky field on a spiral arm of Messier 81. With a beam of 5''≃90pc, we identify 6 giant molecular cloud complexes with virial masses of ≃10⁶M_☉, including one associated with a giant HII region. The deduced N(H₂)/I_CO ratios are about 3 times larger on average than those measured near the solar neighborhood, suggesting that the complexes are not self-gravitationally bound except, possibly, for the complex associated with the giant HII region; they could be the average of several clouds of mass a few 10⁵M_☉ and diameter ≤100pc. The linewidths are very narrow with respect to the measured sizes, so that the size-linewidth relation for M 81 clouds is very different from that in the Milky Way. The narrow linewidths imply smaller virial masses than for Galactic complexes of the same size, and this is consistent with the weaker CO emission from the GMCs in M 81. The low velocity dispersion suggests a lower mean volume density in the cloud and, possibly, a smaller scale height of the molecular gas than in Galactic clouds of the same size. Comparison of the interferometer and single-dish line profiles indicates that, at most, 30% of the single-dish emission in this field is from a widespread distribution of small clouds, and thus the population of molecular clouds is rather different from that in the Milky Way. The H₂ surface density in M 81 is low: although the region studied here is one of the richer molecular regions in the disk, the molecular surface density is much smaller than the interarm regions of M51 for example. The HI gas dominates and can explain most of the extinction seen at optical wavelengths in this field. In some other fields, the HI gas cannot explain the observed extinction, but previous lower resolution observations detected little or no CO there. The present high resolution observations imply that the molecular medium in M 81 differs from that in the Milky Way.