Identification of molecular complexes in M81

Abstract

We report about high spatial resolution observations made with the IRAM interferometer of the CO emission from a 1.1x1.1 kpc plane-of-sky field on a spiral arm of Messier 81. With a beam of 5&#39;&#39;(90 pc), we identify 6 giant molecular cloud complexes with virial masses of about 10^6 Msun, 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 10^5 Msun and diameter < 100 pc. 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. We conclude from these observations that the molecular medium in M 81 differs from that in the Milky Way.