ROTATION OF GIANT MOLECULAR CLOUDS AND HYDRODYNAMIC EFFECTS IN THE GALACTIC DISC

Observational data indicate that giant molecular clouds have spin rota tion with angular momenta of up to about 100 km s(-1) pc, per unit mas s. The spin angular momenta of the clouds may be due to the orbital ro tation of the gas in the Galactic disc where the clouds form, as was s uggested by Blitz. We discuss here another possible mechanism for the origin of the shin rotation of the clouds, which is related to hydrody namical 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 c ontributions dominates in various areas of the Galactic disc. It is pr obable that the effect of vorticity generation might be responsible at least for the retrograde (antiparallel to the spin of the Galaxy) rot ation of giant molecular clouds in all areas where the rotation curve of the Galaxy is rising or flat.