On the nature of supersonic turbulence in Giant HII Regions.

New FLAMES/GIRAFFE public data allows to revisit the issue of the origin of the supersonic turbulence inferred in the ionized gas of Giant HII Regions using the prototypical 30~Doradus nebula in the LMC as a guide. We find that the velocity width of the integrated H$\alpha$ line-profile of 30~Doradus can only be explained by the motion of macroscopic parcels of gas driven by the gravitational potential of the stars and gas with a significant contribution of stellar winds. At all positions within the nebula we find that an additional very broad ($\sigma=40-50$~\kms) unresolved component is required to fit the extended wings of the profiles. We find that fitting two Gaussian components to the integrated emission-line profiles provides a robust way of separating the contributions of stellar winds and gravity. Assuming an effective radius of 10pc for the nebula we thus infer a dynamical mass of $\sim10^6$~\msun~ for 30Dor. Our analysis of published observations of the second largest Giant HII Region in the Local Group, NGC~604, shows that our results for 30~Doradus apply to GHR as a class.