The abundance and chemical evolution of nitrogen in spiral galaxies.

Abstract

The character of nitrogen processing in spiral galaxies is studied in this dissertation. Of particular interest are questions of how the (N/O) ratio changes over time as a result of perturbations of environmental parameters, as well as the importance of primary vs. secondary nitrogen generation and the regimes where one may be the preferred method. A robust numerical chemical evolution code (NICE) was written to model the change in elemental ratios during galactic chemical processing. This code is consistent with standard observational constraints. A new method is developed for the calculation of (N/O) abundances in the absence of observed temperature-diagnostic emission lines. New (N/O) abundances are derived for previously observed HII regions in spiral and dwarf galaxies, and the trends noted in the observations are modeled with the numeric code NICE. I conclude it is likely that early-type spirals once had a higher rate of infalling material relative to late-type galaxies, resulting in both a higher (N/O) ratio as well as a lower gas fraction during later epochs. NICE models also suggest that the star formation rate was suppressed in the extremely metal-poor stages of galaxy chemical processing, as shown by the model fits to the I Zw 18 regions as well as a highly redshifted primeval galaxy. Primary nitrogen production is only realized in stars of 4-8 solar masses, so that this is the first source of nitrogen after an episode of star formation. This is seen in both the observations and the models of low-metallicity dwarf galaxies. At later times, secondary nitrogen is released by stars in the lower mass range (1-4 solar masses), contributing to the steeper slope seen in (N/O) vs. O/H for the more chemically advanced spiral galaxies.