An Evolutionary Sequence of Expanding Hydrogen Shells in Galaxy Discs

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 present observational evidence within a energetically consistent framework which strongly supports the scenario in which OB associations do produce the giant HI shells. Methods. Using Fabry-Perot scanned Hα emission line mapping of nearby galaxy discs, we have detected, in all the H  regions where the observations yield sufficient angular resolution and S:N ratio, dominant Hα shells with radii a few tens of pc, expanding at velocities of 50-100 km s−1, and with gas masses of 104 − 10M⊙. In previous studies, we found that stellar winds alone can account for the energetics of most of the Hα shells, which form initially before the stars explode as SNe. We have applied a simple dynamically consistent framework in which we can extrapolate the properties of the observed Hα shells to a few 107yr after the formation 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 Hα emitting shells are generic progenitors of the HI shells. Results. The results are in good agreement with the ranges of masses (∼10M⊙), velocities (up to ∼20 km s−1), and diameters (up to ∼500 pc) of representative HI shells observed in nearby galaxies. The combined effects of stellar winds, acting during the first few 106yr, and SN explosions, ”switching on” subsequently, are required to yield the observed parameters.