Tracing galaxy evolution through the reconstruction of their star formation histories
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This thesis studies the star formation histories (SFHs) of galaxies in order to understand the nature of 'quenching.' The suppression of star formation, i.e., quenching, over the history of the Universe results in a growing population of quiescent galaxies. The physical processes governing how and why galaxies are quenched remain unknown. Observations show that massive galaxies, and those in dense cluster environments, make up a great proportion of the quiescent population. A number of theories have been proposed to explain both the mass dependence and environmental dependence of this population. Through studying the ages and stellar properties of the quiescent galaxies, the predictions of these theories can be tested. The first section of this thesis concentrates on the differences between galaxies in isolated ('field') or cluster environments as part of the Gemini Observations of Galaxies in Rich Early ENvironments (GOGREEN) survey. Leveraging multi-wavelength and spectroscopic observations of 331 quiescent galaxies at 1 < z < 1.5, the data is fit to model spectral energy distributions (SEDs) to infer the SFHs and properties of the individual galaxies. In looking at the age trends between galaxies of different stellar masses, it is confirmed that more massive galaxies show evidence of earlier formation times, while lower mass galaxies exhibit more diverse SFHs. This result supports the paradigm of mass-dependent galaxy evolution. The novel result of this work was that any age difference between cluster and field galaxies was subtle; at fixed stellar mass cluster galaxies are <0.5 Gyr older. Putting this result in the context of two simple quenching models rules out two proposed quenching scenarios: i) environmental quenching post-infall, and ii) a primordial quenched population among cluster galaxies. This is distinctly different from local clusters, for which the majority of the quiescent population is consistent with having been environmentally quenched upon infall. Our results suggest that the quiescent cluster population at z > 1 is driven by different physical processes than those at play at z = 0. The second section of this thesis focuses instead on the detailed characterization of a single galaxy, the Ultra Diffuse Galaxy (UDG) Dragonfly 44 (DF44), whose curious set of properties is inconsistent with theoretical models of UDG formation. In fitting broadband photometry with high signal-to-noise and high-resolution optical spectroscopy with SED models, the detailed stellar properties and SFH of this galaxy are investigated. The precision of the observations required a careful assessment of the SED models, where the conclusion was that DF44 formed between 7.2–12.9 Gyr ago. Regardless of whether DF44 is old or very old, the SFHs imply early formation and rapid quenching. This result in context with its large size, kinematics, stellar population properties, and its environment, challenges conventional theories of galaxy evolution. The implication is that current theoretical models are missing the true diversity of galaxy formation and evolution. The third section evaluates the assumptions of the SED fitting procedure. Modelling the SFHs of galaxies is an ill-defined problem, where the results are subject to a number of prior assumptions for what SFHs are more realistic. Modern SED fitting models make a number of these assumptions specific, which provides flexibility in studying diverse samples of galaxies. Without observational constraints to qualify such assumptions, however, the results can simply reflect the assumptions. A mock dataset of quiescent galaxies modelled after the GOGREEN sample is constructed in order to investigate the influence of the SFH prior to the results of that study. A statistical framework is used to infer the distribution of properties among a galaxy population, which in principle can mitigate unphysical assumptions made in SED-fitting. This work highlights the challenges involved in studying star formation timescales of old galaxies, and the nuances of SED-fitting procedures which can lead to spurious results.
Cite this version of the work
Kristi Webb (2023). Tracing galaxy evolution through the reconstruction of their star formation histories. UWSpace. http://hdl.handle.net/10012/19473