Star formation triggering mechanisms in dwarf galaxies: The far-ultraviolet, H alpha, and HI morphology of Holmberg II

Far-ultraviolet (FUV), H alpha, and H I observations of dwarf galaxy Holmbe rg II are used to investigate the means by which star formation propagates in galaxies lacking global internal triggering mechanisms such as spiral de nsity waves. The observations trace the interaction between sites of massiv e star formation and the neutral and ionized components of the surrounding ISM in this intrinsically simple system. Both local and large-scale trigger ing mechanisms related to massive star formation are seen, suggesting that feedback from massive stars is a microscopic process operating in all galax ies to a certain degree. The data emphasize the importance of local conditi ons in regulating star formation from evidence such as massive stars inside ionized shells, compact H II regions surrounding aging clusters, and stars formed in chains of progressing age. Surface brightness profiles show that current activity correlates with the time-averaged level of past star form ation at a given radius demonstrating a reliance on local conditions. Large -scale triggering by H I shells is supported by observations of progenitor populations as well as secondary sites of star formation associated with th eir dense rims. Analysis of the energy available from massive stars inside H I shells indicates that energy deposited into the ISM from supernovae and stellar winds is sufficient to account for the H I morphology. Ages of ind ividual star-forming regions are derived using B, H alpha, and FUV photomet ry and show both older, diffuse FUV regions and younger, compact H II regio ns. The distribution of ages is reconciled with the H I morphology, showing a clear preference of young regions for areas of dense H I and old regions for H I voids. Global kinematical properties may also play a role in the s tar formation process since differences in the rotation characteristics of the neutral gas disk correlate with differences in triggering mechanisms. L arge-scale feedback from massive stars is shown to operate in regions that lack differential shear in the gas disk.