Rotation of giant molecular clouds and hydrodynamic effects in the Galactic disc

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Abstract
Observational data indicate that giant molecular clouds have spin rotation with angular momenta of up to about 100 km s–1 pc, per unit mass. The spin angular momenta of the clouds may be due to the orbital rotation of the gas in the Galactic disc where the clouds form, as was suggested by Blitz. We discuss here another possible mechanism for the origin of the spi rotation of the clouds, which is related to hydrodynamical processes in the interstellar gas. We show that the effect of vorticity generation arising from the interaction of the interstellar gas with large-scale spiral shock fronts can contribute significantly to the spin rotation of giant molecular clouds formed from the gas. We try to find observational criteria that might indicate which of the contributions dominates in various areas of the Galactic disc. It is probable that the effect of vorticity generation might be responsible at least for the retrograde (antiparallel to the spin of the Galaxy) rotation of giant molecular clouds in all areas where the rotation curve of the Galaxy is rising or flat.

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