2003ApJ...583..280B

Principal component analysis of ¹²CO (J=1-0) emission is used to diagnose the presence of large-scale, global velocity fluctuations in molecular clouds. We search for, and find, large-scale flows of atomic material in which the globally turbulent molecular clouds are embedded. This is consistent with the picture of molecular clouds existing as short-lived, turbulent density fluctuations within larger scale atomic flows. Large-scale driving of turbulence in molecular clouds, by converging flows of atomic material, can reconcile recent numerical determinations of the dissipation rate of turbulent energy with the observations, that otherwise, for small-scale driving, leads to unacceptably high cloud luminosities. On the other hand, lack of large-scale shock signatures in the molecular gas, as expected for large-scale driving, may, if not due to an observational bias, require that the large-scale flows are themselves driven by energy injection occurring on smaller scales within molecular clouds.