The impact of star formation on the interstellar medium in dwarf galaxies

Abstract

This thesis is an observational study of the impact of star formation on the interstellar medium. Emission from the ionized component of the interstellar gas is used to measure both the kinematics and the physical properties of the gas in 14 dwarf galaxies. The galaxies examined show emission from ionized gas outside the HII regions. This warm ionized medium has a substantial power requirement and often shows arcs and filaments on scales exceeding 100 pc. To determine the mix of physical processes exciting the gas, I measure optical emission-line ratios averaged over scales of 30-150 parsecs at thousands of locations within the galaxies. I find that, relative to HII regions, the spectrum of the diffuse ionized gas has stronger lines from low-ionization states of oxygen, nitrogen, and sulfur and weaker lines from highly ionized atoms. The HII-DIG spectral transition defines a narrow sequence in diagnostic-line-ratio diagrams which is distinct from the conventional HII-region excitation sequence. Photoionization modeling demonstrates that the HII-DIG sequence is driven primarily by a decrease in the relative density of ionizing photons to atoms--consistent with ionization by distant stellar clusters. The strength of the line emission from ionized He relative to that from ionized hydrogen implies stars of mass greater than 35 M(⊙) contribute to the ionizing continuum. A second excitation process, shocks with speeds from 60-100 km s⁻¹, is shown to contribute up to 50% of the emission from the lowest surface brightness gas. High-resolution, longslit spectra reveal organized gas flows on scales ranging from the resolution limit, about 20-100 km s⁻¹, to galactic-scale dimensions, roughly 1 kpc. Many of the expanding shells detected kinematically are coincident with arcs or filaments in the galaxy images, and the geometry of the larger bubbles is often polar rather than spherical. The simplest dynamical models for their growth imply ages from 1-20 Myr and require energies from 1 to more than 6000 supernovae. The extent, mass, and rotation of the neutral gas are compiled from the literature and used to argue that many of these bubbles breakthrough the HI disk and form galactic winds. A detailed study of I Zw 18 suggests such winds could have a strong influence on the chemical evolution of dwarfs. The total mass of gas escaping the galaxies is, however, not yet well-constrained and may be quite small.