UC Irvine

This thesis presents a computational study of the formation and evolution of Milky-Way-mass galaxies using FIRE-2 cosmological zoom-in simulations. All simulated galaxies experience an early period of bursty star formation that transitions to a late-time steady phase of near-constant star formation. Stars form at different time have very different kinematic properties, morphologies, and chemical abundances. We classify stars in the central galaxy as bulge, thick-disc, and thin-disc according to their orbital properties and discover that these components are assembled in a time-sequence from early to late times, respectively. Stars born during the early bursty phase have more radial orbits and they evolve into a central spheroidal structure at z=0, while stars form during the late-time steady phase have more circular orbits and thin-disc-like morphology. Thick-disc stars form during a transitional period, mostly before the star formation settles down. Thus, we find a strong correlation between the age of thick-disc population at z=0 and the transition time. We also find that galaxies with earlier transition from bursty to steady star formation have higher thin-disc fractions at z=0. Bursty star formation at early time also triggers a significant amount of strong outflow events. During starbursts, the instantaneous fraction of stars born in outflows can be as high as ~ 20 - 50%. These outflow stars contribute substantially to the in-situ stellar halo. Statistically, stellar populations form at different times tend to maintain their disc/bulge-like kinematics, with only moderate evolution over time. Mergers do alter kinematics, but play a secondary role in ''disc-settling'' process. They do not destroy the thin disc nor change broad trend between star formation transition time and thin/thick-disc/bulge properties. If our simulations are representative of the Universe, then stellar archaeological studies of the Milky Way can provide a window into past star-formation modes in the Galaxy. Current age estimates of the Galactic thick disc would suggest that Milky Way transitioned from bursty to steady phase > 6.5 Gyr ago; prior to that time the Milky Way likely lacked a stable thin disc and underwent intense outflow activities.