Assessing and understanding diversity in galaxy star formation histories

Galaxy star formation histories (SFHs) form a central thread of the cosmological narrative. Assessing and understanding them is therefore a central mission of the study of galaxy evolution. Although an ever-better picture is emerging of the build-up of the stellar mass of the average galaxy over time, the relevance of this track to the growth of individual galaxies is unclear. Largely, this ambiguity is due to the availability of only loose, ensemble-level constraints at any redshift appreciably greater than zero. In this thesis, I outline how these constraints — principally the cosmic star formation rate density, stellar mass function, and the star formation rate/stellar mass relation — shape empirically based SFH models, especially in terms of the diversity of paths leading to a given end-state. Along the way, I show that three models propose very different answers to this question, corresponding (largely) to three different interpretations of the scatter in instantaneous galaxy growth rates at fixed stellar mass. I describe how these interpretations affect one's stance on the fundamental importance of so-called galaxy "bimodality" and quenching mechanisms, the influence of environment, and the role starbursts play in galaxy evolution. Ultimately, I conclude that there is insufficient evidence at present to select one interpretation over all others, but suggest that the situation might soon be resolved by upcoming observations that could clearly identify which model (or hybrid) is the most accurate description of galaxy growth