Stellar population synthesis for the future

Knowledge of the physical properties of galaxies is essential to identify the processes responsible for the emergence and evolution of galaxies. Stellar population synthesis (SPS) is the principle technique that links the observable characteristics of galaxies to their physical properties, including their star formation and chemical enrichment histories, and dust content. This thesis consists of a new approach to and critical evaluation of the SPS technique. In the following chapters I assemble the necessary ingredients for SPS, assess their relative uncertainties, and demonstrate how these uncertainties propagate into the derived physical properties of galaxies. I elucidate the difficulty in robustly constraining galaxy formation models by translating model outputs into observables, and instead offer a more promising approach toward constraining such models. I also consider constraints on SPS models provided by observations of star clusters throughout the Local Group. Comparison to these data not only uncovers several significant model deficiencies but also highlights the large regions of parameter space (in age and metallicity) not covered by the data. I also use observations of star-forming galaxies both in the local Universe and at z ∼ 1 to assess and constrain models for dust attenuation, paying particular attention to the possible presence of the strong absorption feature at 2175A seen in the extinction curve of our Galaxy. In the local sample, I report evidence for the 2175A feature, which constitutes the first such detection at z ∼ 0, and implies that previous work, which ignores this feature, has mis-estimated the attenuation in the near-UV. This work builds the framework for a more quantitative and precise estimate of the physical properties of galaxies, and in addition allows for the exploration of a wide variety of non-canonical SPS effects, including the 2175A absorption feature, mass--loss along the red giant branch, blue straggler stars, the morphology of the horizontal branch, thermally-pulsating asymptotic giant branch stars, and alternative stellar initial mass functions.