Celestial Dragons and Brushing Teeth
Black, William
2023
Abstract
This dissertation delves into two domains. The first domain is cosmology and astrophysics, investigating galaxy occupation of dark matter halos as well as characterization of the star-forming and quiescent galaxy populations in photometric color space. The second domain covered is physics education research, investigating student grade gains due to practice study. Chapter 2 measures how galaxies occupy the dark matter halos of the Buzzard synthetic galaxy catalog, finding several divergences from observations. In characterizing halo richness as a function of mass, we find a significantly shallow slope and a decreasing scatter with increasing mass. We then characterize the fraction of red sequence (RS) galaxies in halos, investigating its dependence on richness, mass, redshift, and radius. Surprisingly, we find that red fraction follows an inverted mass dependence—of decreasing with increased halo mass; this leads to high-mass halos lacking RS galaxies. We also find that—again, contrary to expectations—red fraction increases with redshift. We also investigate radial density profiles, and find a surprising deficit of galaxies near one-tenth the virial radius. Chapter 3 formally introduces the Red Dragon algorithm, detailing its selection of galaxies in Buzzard. Red Dragon is a multivariate error-corrected Gaussian Mixture Model (GMM). This results in smooth characterizations of both RS as well as blue cloud (BC) galaxies across time, thereby avoiding discontinuities inherent in swapping RS selection colors. We find a quiescent (low specific star formation rate) selection accuracy of over ninety percent, indicating a successful characterization of the two populations. Chapter 4 uses Red Dragon to characterize the RS and BC in the COSMOS2015 dataset. This deep field allows for high-precision redshift estimates, which give strong measures of population characteristics (out to roughly redshift 1.5). We again find high selection accuracy (over roughly 90%) of the quiescent population. With this characterization, we measure temporal evolution of median galactic specific star formation rate as well as galactic age. Chapter 5 then turns to the domain of physics education research. We first condition final course grade on mean ACT/SAT math score T, subtracting out the expected grade, in order to account for incoming math proclivity. Using data from the online study service Problem Roulette (PR), we measure grade gains due to practice study, finding that maximum gains were roughly .77 grade points, moving from no study to studying 1000 PR questions over the term (roughly ten per day). These gains persisted at any fixed math score. After modeling expected grade using T and study volume, we then investigate divergences from this expectation for various demographic groups. We find that students whose parents did not earn a college degree earned .27 grade points below expectations (over 99.99999% confident non-zero) and find that underrepresented minority students earned .14 grade points below expectations (99.98% confident non-zero). All divergences from expectations subceeded maximal grade gains of study. Residual scatter remains comparable to maximal study gains, implying that the model is far from deterministic: individual variation trumps mean trends. Our findings can help motivate student study habits and help teachers identify which students may especially need such encouragement.Deep Blue DOI
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Physics Cosmology Astrophysics Physics Education Research Galaxies Photometry
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