UC San Diego

The interstellar medium (ISM) consists of the gas and dust in between stars. A critical component that regulates the physics of the ISM is its chemical abundance (i.e. metallicity). Due to the stellar nucleosynthetic origin of metals, a galaxy's metallicity reflects its history of chemical enrichment from stars. A number of astrophysical phenomena (e.g. supernovae, stellar feedback) can mix and redistribute metals throughout the ISM. To distinguish the impact of these competing effects, it is crucial that we observationally constrain the large and small scale variations of metallicity across the ISM. In this dissertation, we investigate the connection between H II region electron temperatures, important for determining metallicities, and the physical properties of the ISM. Additionally, we investigate the source of ionizing photons in an extremely low metallicity galaxy.

In Chapter 2, we compare multi-ion H II region electron temperature measurements to several ISM properties such as electron density, ionization parameter, and molecular gas velocity dispersion. We measure anomalously high doubly-ionized oxygen temperatures from auroral lines in regions with high molecular gas velocity dispersion and low ionization parameter. These anomalous temperature measurements may be explained by the presence of low-velocity shocks.

In Chapter 3, we present H II region metallicities measured using nitrogen and sulfur temperature sensitive lines. We find that temperatures inferred from doubly-ionized sulfur emission lines are impacted by temperature fluctuations. We measure a strong correlation between metallicity and sulfur-to-oxygen and nitrogen-to-oxygen abundance ratios. Furthermore, we report correlations between the scatter in the nitrogen-to-oxygen ratio with the hardness of the ionizing spectrum and with the molecular gas velocity dispersion.

In Chapter 4, we present observations of the very metal-poor galaxy I Zw 18 that reveal two very high ionization regions. The two regions lie along an axis which intercepts the position of I Zw 18's Ultra-luminous X-ray (ULX) source. We explore whether the ULX could power the two regions via jets and/or beamed X-ray emission, and other alternative sources.