Dynamical Constraints on Massive Star Formation

Extragalactic HII regions and their ionizing star clusters are the most prominent signposts of recent starbirth activity in galaxies. In this paper, we present optical measurements of nearby extragalactic HII regions with the aim of investigating possible relationships between the newborn ionizing clusters and their dynamical environment. Evidence for variations in the measured HII region properties (size, Ha luminosity, and Ha equivalent width) are presented as a function of galaxy type and of position within individual galaxies. These variations, in turn, are compared with the corresponding kinematic and dynamical variations. The apparent sensitivity of the ionizing activity to the dynamical environment suggests that the total masses and upper stellar mass limits of the ionizing clusters are somehow constrained by the ambient tidal stresses and shear flows within the galaxies. We speculate that regions of high tidal stress and kinematic shearing tend to suppress the formation and maintenance of giant molecular clouds, thereby hindering the creation of giant clusters and massive stars therein.