Aims. Large HI shells, with diameters of hundreds of pc and expansion velocities of 10-20 km s-1 have been detected in their hundreds in the Milky Way and are well observed features of local gas rich galaxies. These shells could well be predicted as a result of the impact of OB associations on the ISM, but doubt has been cast on this scenario by the apparent absence of OB stars close to the centres of a large fraction of these shells in recent observations of the SMC. Here we presetn observational evidence within an energetically consistent framework which strongly supports the scenario in which OB associations do produce the giant HI shells.
Methods. Using Fabry-Perot scanned Halpha emission line mapping of nearby galaxy discs we have detected, in all the HII regions where the observations yield sufficient angular resolution and S:N ratio, dominant Halpha shells with radii a few tens of pc, expanding at velocities of 50-100 km s-1, and with gas masses of 10*4 -10*5 Mo. In previous studies we found that stellar winds alone can account for the energetics of most of the Halpha shells, which form initially before the stars explode as SNe. We have applied a simple dynamically consistent framework in which we can extrapolate properties of the observed Halpha shells to a few x 10*7 years after the formatino of the OB stars. The framework includes the dynamical inputs of both winds and SNe on the surrounding ISM. The results give quantitative statistical support to the hypothesis that the Halpha emitting shells are the generic progenitors of the HI shells.
Results. The results are in good agreement with the ranages of masses ( some 10*6 Mo) velocities ( up to some 20 km s-1) and diameters ( up to some 500 pc) of representative HI shells observed in nearby galaxies. the combined effects of stellar winds, acting during the first few x 10*6 yr and SN explosions, "switching on " subsequently, are required to yield the observed parameters.