VINTERGATAN – II. The history of the Milky Way told by its mergers

Using the VINTERGATAN cosmological zoom simulation, we explore the contributions of the in situ and accreted material, and the effect of galaxy interactions and mergers in the assembly of a Milky Way-like galaxy. We find that the initial growth phase of galaxy evolution, dominated by repeated major mergers, provides the necessary physical conditions for the assembly of a thick, kinematically hot disk populated by high-[$\alpha$/Fe] stars, formed both in situ and in accreted satellite galaxies. We find that the diversity of evolutionary tracks followed by the simulated galaxy and its progenitors leads to very little overlap of the in situ and accreted populations for any given chemical composition. At a given age, the spread in [$\alpha$/Fe] abundance ratio results from the diversity of physical conditions in VINTERGATAN and its satellites, with an enhancement in [$\alpha$/Fe] found in stars formed during starburst episodes. Later, the cessation of the merger activity promotes the in situ formation of stars in the low-[$\alpha$/Fe] regime, in a radially extended, thin and overall kinematically colder disk, thus establishing chemically bimodal thin and thick disks, in line with observations. We draw links between notable features in the [Fe/H] - [$\alpha$/Fe] plane with their physical causes, and propose a comprehensive formation scenario explaining self-consistently, in the cosmological context, the main observed properties of the Milky Way.