IDENTIFICATION OF MOLECULAR-COMPLEXES IN M-81

We report about high spatial resolution observations made with the IRA M Plateau de Bure interferometer of the (CO)-C-12(J = 1 --> 0) emissio n from a 1.1 x 1.1 kpc plane-of-sky field on a spiral arm of Messier 8 1. With a beam of 5 '' approximate to 90 pc, we identify 6 giant molec ular cloud complexes with virial masses of approximate to 10(6) M., in cluding one associated with a giant HII region. The deduced N (H-2)/I- CO ratios are about 3 times larger on average than those measured near the solar neighborhood, suggesting that the complexes are not self-gr avitationally hound except, possibly, for the complex associated with the giant HII region; they could be the average of several clouds of m ass a few 10(5) M. and diameter less than or equal to 100 pc. The line widths are very narrow with respect to the measured sizes, so that the size-linewidth relation for M 81 clouds is very different from that i n the Milky Way. The narrow linewidths imply smaller virial masses tha n 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 disper sion 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 t his field is from a widespread distribution of small clouds, and thus the population of molecular clouds is rather different from that in th e Milky Way. The H-2 surface density in M 81 is low: although the regi on studied here is one of the richer molecular regions in the disk, th e molecular surface density is much smaller than the interarm regions of M 51 for example. The HI gas dominates and can explain most of the extinction seen at optical wavelengths in this field. In some other fi elds, the HI gas cannot explain the observed extinction, but previous lower resolution observations detected little or no CO there. The pres ent high resolution observations imply that the molecular medium in M 81 differs from that in the Milky Way.