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Astronomical Applications Department, U.S. Naval Observatory - thesis (Page 13)

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Astronomical Applications Department, U.S. Naval Observatory - thesis
Abstract
Far-ultraviolet FUV, H , 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 den-
sity 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 for-
mation 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 pro les 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 condi-
tions 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 su cient to account for the HI morphol-
ogy. Ages of individual star forming regions are derived using B, H , and FUV
photometry and show both older, di use 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 di erences in the rotation characteristics of the neutral gas disk correlate
xii

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