Unraveling galaxy evolution using numerical simulations

One of the primary concerns in galaxy evolution is how galaxies form their stars: what keeps that star formation going over cosmic time, and what causes it to stop in a processes called "quenching". Galaxies with mass similar to our own Milky Way occupy a sweet spot between abundance and brightness that makes them easy to find in the sky, and such galaxies also populate a transitionary regime in behavior that make them interesting for studying galaxy evolution. Numerical modeling-from semi-analytic models to numerical simulations-are valuable tools for understanding the multiple intersecting physical processes that drive galaxy evolution. These processes act both within and around individual galaxies such that numerical models must necessarily encompass a range of spatial and temporal scales. Multiple approaches are commonly used in order for this modeling to be physically insightful. In this dissertation I will present my efforts to unravel the mechanisms of galaxy evolution affect Milky Way-like galaxies using a variety of numerical models.Addressing the issue of what causes galaxies to stop forming stars, I first investigate an unusual population of galaxies called the "break BRDs" (Tuttle and Tonnesen 2020). Within the dominant framework for galaxy quenching, galaxies first stop forming stars in their centers and later in their outskirts. This is the "inside-out" quenching paradigm. The break BRD galaxies possess observational markers that run counter to this narrative. We used the Illustris TNG cosmological simulation(Pillepich et al. 2018b) to find a set of simulated galaxies that are analogous to the observed breakBRDs in order to better understand their evolution. We found that the breakBRD analogues are galaxies that ultimately become fully quenched, but found no clear cause for the "outside-in" modality. This is not the dominant channel for quenching in the IllustrisTNG simulation, but roughly 10% of quiescent galaxies with 10 < log10 (M⁸́₇/M⁽́₉) < 11 had centrally-concentrated star formation similar to the breakBRD analogues.As to what keeps galaxies forming their stars, I used a set of idealized simulations of Milky Way-like galaxies to study the interactions of the circumgalactic medium (CGM) and its host galaxy. The CGM is an extended volume of gas that accounts for about half of the baryonic matter in agalaxy's dark matter halo. This gas is also "multiphase," containing gas at a wide range of densitiesand temperatures. It may therefore function as a reservoir from which gas may cool, condense,and accrete onto the host galaxy where it can eventually drive star formation and stellar feedbackprimarily via Type II supernovae. This cycle of condensation and feedback may self-regulatethe overall star formation rate of a galaxy. Our idealized simulations include both the CGM andexplicit formation of stars but find that stellar feedback can drive outflows that disrupt the CGMwith large, hot, low-density cavities. This is true even after we adjust the stellar feedback efficiencyto accommodate the "settling" of the initial conditions. We therefore conclude that the picture ofstar formation self-regulation in Milky Way-like galaxies is missing physical processes at the edgeof the galaxy halo that work in tandem with accretion of CGM gas and stellar feedback.The CGM is typically observed via absorption spectra that contain features from numerous metal ions. In order to better compare the simulated CGM with observations, most simulations need to be post-processed to derive similar information as that extracted from spectra. Therefore, I also present preliminary work quantifying the uncertainties inherent to this post-processing. The results herein focus on the assumption that metals in the CGM follow the abundance pattern of our Sun, which is not physically well-reasoned. We derive plausible alternative abundance patterns using chemical evolution modeling and apply these to a post-processing of the FOGGIE cosmological zoom simulations (Peebles 2020; Simons et al. 2020). We find that adopting a non-Solar abundance affects the column density of CGM absorbers of about ℗ł1 dex.Finally, I present future research directions for all the projects described herein. These include investigating the CGM of the breakBRD analogues from IllustrisTNG, outlining additions to ouridealized galaxy simulations that may address the issue of disruptive outflows, and both scaling up our existing uncertainty quantification project as well as including the additional source of uncertainty, ionizing radiation.