The Outermost HII Regions of Nearby Galaxies.

Abstract

The outlying HII regions of nearby galaxies provide a unique opportunity to probe the star-formation history, evolution, and origin of the gas in the outskirts of galaxies. Following a systematic search of a sample of ``normal" gas-rich galaxies derived from the NOAO Survey for Ionization in Neutral Gas Galaxies (SINGG), and a separate sample of interacting, disturbed, or extended HI-disk galaxies (HI Rogues), I have catalogued and confirmed over 75 outlying HII regions located far beyond the main optical components of galaxies (r > r25). In this thesis, I use outlying HII regions to address several fundamental questions about the nature of star formation in the low-density gas outside of galaxies. In particular, I determine the frequency with which outlying HII regions occur in gas-rich galaxies, the properties of their resolved, underlying stellar populations, and the oxygen abundances of the outermost, low-density gas in which they form. First, I use a self-developed automated algorithm to search for HII regions beyond projected optical radii in an unbiased sample of gas-rich galaxies. I find the overall frequency of outlying HII regions in the SINGG sample of gas-rich galaxies is 8 - 11% when I correct for background emission-line galaxy contamination (~ 75 % of emission-line point sources). I study the resolved stellar populations of several of these outlying HII regions using broadband HST ACS/HRC images. To the resolution and faintness limits of the HST images, outlying OB associations have the same stellar populations as Galactic OB associations, and appear to be dissolving on rapid timescales. Finally, I present optical emission-line spectra for outlying HII regions in the extended and/or disturbed neutral gas surrounding 14 nearby star-forming, gas-rich galaxies with a wide range of morphologies. The result that the outer-galaxy gas is consistently enriched to the same level as the inner-galaxy gas has implications for the physical origin of the mass-metallicity relation for gas-rich dwarf galaxies, as well as the chemical evolution that takes place within galaxies over their lifetimes. This work represents significant progress for understanding star formation in low-density conditions and its role in galaxy evolution.