Unraveling Galaxy Evolution Using Numerical Simulations

One of the primary concerns in galaxy evolution is how galaxies form their stars: what keeps thatstar 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 unusualpopulation of galaxies called the “breakBRDs” (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 breakBRD galaxies possess observa- tional markers that run counter to this narrative. We used the IllustrisTNG 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 (?∗/M⊙) < 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 MilkyWay-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 a galaxy’s dark matter halo. This gas is also “multiphase,” containing gas at a wide range of densities and 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 feedback primarily via Type II supernovae. This cycle of condensation and feedback may self-regulate the overall star formation rate of a galaxy. Our idealized simulations include both the CGM and explicit formation of stars but find that stellar feedback can drive outflows that disrupt the CGM with large, hot, low-density cavities. This is true even after we adjust the stellar feedback efficiency to accommodate the “settling” of the initial conditions. We therefore conclude that the picture of star formation self-regulation in Milky Way-like galaxies is missing physical processes at the edge of 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 numerousmetal 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 ±1 dex.Finally, I present future research directions for all the projects described herein. These includeinvestigating the CGM of the breakBRD analogues from IllustrisTNG, outlining additions to our idealized 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.