Galaxy evolution: the relationship between structure, star formation, and environment
I have conducted two separate observational tests of the current theoretical paradigm for galaxy formation and evolution, hierarchical galaxy formation in a Cold Dark Matter dominated universe. The first project is a spectroscopic study of nineteen low surface brightness galaxies observed with the 9.2m Hobby-Eberly Telescope. I determine redshifts to all the galaxies, and report both emission line and absorption line index strengths, where appropriate. Correlations are found between the gas-phase metallicity indicators and the stellar indices which are sensitive to both metallicity and age. I interpret the star formation and chemical evolutionary histories of these galaxies using a combination of simple stellar population models and nebular emission models. The correlations observed between emission and absorption measurements place strong constraints on possible star formation histories. We find that low surface brightness galaxies are a diverse class, with properties as broad as those exhibited by the full range of high surface brightness galaxies. The second project is a study of the ages in early-type field galaxies, and a comparison with the ages found for a sample of cluster galaxies. I present preliminary results and interpretations based on new stellar population modelling methods. The new model explicitly accounts for age, metallicity, and α–abundance ratios for the stellar population, by making predictions for low resolution line indices. I measure the ages, metallicities, and α–abundance as a function of radius, from the center of each galaxy to beyond one effective radius. The gradients are consistent with younger, more metal rich stellar populations in the centers of most of the galaxies, but a constant α–abundance ratio within any one galaxy. I have generated an all-sky catalog which measures the local density of galaxies around every galaxy at cz<10,000 km s −1 . This facilitates the creation of low redshift field galaxy samples for follow-up study, in particular to study the effects of environment on galaxy evolution. I attempt to quantify the effects on the catalog of the varying spectroscopic survey completeness across the sky, and compare my local density measures with published field galaxy samples. The full catalog (42,024 galaxies, with ∼ 50 parameters per galaxy) is included in the electronic version of this dissertation.