Star formation triggering mechanisms revealed by far-ultraviolet, H(alpha), and HI images of dwarf galaxies

Far-ultraviolet (FUV), Halpha, and HI observations of dwarf galaxies Holmberg II, IC2574, and Sextans A are used to investigate the means by which star formation propagates in galaxies lacking global triggering mechanisms such as spiral density waves. The observations trace the interaction between sites of massive star formation and the neutral and ionized components of the surrounding interstellar medium (ISM) in these intrinsically simple systems. Both local and large-scale triggering mechanisms related to massive star formation are seen, suggesting that feedback from massive stars is a microscopic process operating in all galaxies to a certain degree.; The data emphasize the importance of local conditions in regulating star formation from evidence such as massive stars inside ionized shells, compact HII regions surrounding aging clusters, and stars formed in chains of progressing age. Surface brightness profiles show current activity correlates with the time averaged level of past star formation at a given radius demonstrating a reliance on local conditions. Normalized star formation rates show no dependence on global conditions in comparisons with global properties such as the gas fraction. Large-scale triggering by HI shells is supported by observations of progenitor populations and secondary sites of star formation on the dense HI rims. Analysis of the energy available from massive stars inside HI shells indicates energy deposited into the ISM from supernovae and stellar winds is sufficient to account for the HI morphology. Ages of individual star forming regions are derived using B, Ha, and FUV photometry and show both older, diffuse FUV regions and younger, compact HII regions. The distribution of ages is reconciled with the HI morphology, showing a clear preference of young regions for areas of dense HI and old regions for HI voids. Global kinematical properties may also play a role in the star formation process since differences in the rotation characteristics of the neutral gas disk correlate with differences in triggering mechanisms. Large-scale feedback from massive stars is shown to operate in regions that lack differential shear and significant random motions in the gas disk.