Rising from the Ashes: How the Milky Way Got Its Scars

Abstract

Understanding ancient events in the Milky Way's history is essential for building a consistent picture of galaxy formation and evolution, bridging stellar and chemical evolution models with the extragalactic population. Our galaxy experienced three simultaneous, transformative events 8 Gyr ago: our last major merger, the formation of a bar, and a transition chemical enrichment leading to an alpha-element bimodality. The history of these events is encoded in the present-day observable properties of stars: dynamics (positions and velocities), chemical compositions, and ages. In this thesis, we will make use of numerical simulations tailored to understanding how these historical processes of the Milky Way led to its present-day properties. First, we will propose a solution to the mystery of why the bar rotates rapidly when it is old enough to have slowed down by now. Second, we will propose that our last major merger led to the emergence of the alpha-bimodality via a metallicity-dependent halt in star formation. Finally, we will demonstrate that the formation of the bar might have triggered the formation of the bimodality through the same mechanism. This work demonstrates the rich understandings that can be gleaned from idealized simulations specifically tailored to the Milky Way.