VINTERGATAN II: the history of the Milky Way told by its mergers

Abstract: 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.

• The paper presents a high-resolution cosmological zoom simulation that details four evolutionary phases of the Milky Way influenced by major mergers and bursty star formation.
• It links in situ star formation with accreted satellite material to explain the distinct chemical and kinematic properties of the thick and thin disks.
• The findings imply that merger-driven chemical enrichment is key to understanding the observed [$\alpha$/Fe]-[Fe/H] bimodality in the Milky Way.

VINTERGATAN II: The Milky Way's Evolution Through Mergers

The study conducted by Renaud et al. explores the assembly history of the Milky Way galaxy by analyzing and modeling the contributions of both in situ star formation and accreted material from satellite galaxies. Through a cosmological zoom simulation, the research investigates how interactions and mergers with other galaxies have sculpted the structure and chemical abundance patterns observed in the Milky Way today.

The study provides a narrative that outlines how the Milky Way underwent a dramatic evolutionary sequence: from an early phase dominated by major mergers to a subsequent calmer era of self-enrichment and secular evolution. This progression contributed to the establishment of both a chemically and kinematically distinct thick disk, along with a kinematically cold, thin disk.

Key Findings and Methodology

Through rigorous numerical modeling, the authors identify four main phases in the galaxy's evolution:

1. Early Formation and Initial Enrichment: Initially, the Milky Way's ancestor galaxy experienced a series of major mergers, which resulted in a thick disk heavily populated by stars with high [$\alpha$/Fe] formed in both in situ and accreted environments.
2. Starburst Phase ($1.2 < z < 3$): These mergers catalyzed intense, burst-like star formation, rapidly enhancing the [$\alpha$/Fe] ratios and increasing the spread in metallicity of the galactic components.
3. Transition from Thick to Thin Disk ($0.8 < z < 1.2$): As the galaxy transitioned from a merger-driven growth phase to a more quiescent state, a thin, chemically bimodal disk developed, reflecting the cessation of significant starburst activity and a decline in the gas fraction.
4. Late Secular Evolution ($z < 0.8$): In the more stable stellar environment, the thin disk continued growing through internal processes, indicating a shift in star formation regimes from turbulent mergers to more quiescent, gas accretion-driven growth.

The findings demonstrate a nuanced overlap of in situ formed and accreted stellar populations, particularly within the high-[$\alpha$/Fe] sequence, emphasizing the dynamical influence of major mergers. Significantly, the simulation predicts a substantial fraction of the Milky Way's stars have accreted origins, although their chemical signature is distinct from that of in situ stars, characterized by a less pronounced [Fe/H] bimodality.

Implications and Speculative Outlook

The implications of these findings suggest that the observed bimodality in the [$\alpha$/Fe]–[Fe/H] distribution of the Milky Way's stars is inherently tied to historical episode of merger-driven chemical enrichment. The cessation of these interactions allowed for thin disk assembly, in alignment with observed properties of the Milky Way.

Looking towards future developments, enhancing the resolution and incorporating additional physics, such as magnetic fields or more refined feedback mechanisms, could refine these models. Incorporating observational data from forthcoming facilities should offer more vigorous constraints on such simulations, potentially differentiating between scenarios that led to the current stellar configurations.

The VINTERGATAN II project stands as a significant piece of the complex puzzle of galactic archaeology, advocating for further explorations into how the dynamical and chemical legacies of ancient galaxies contribute to current structures. By situating the Milky Way within a broader cosmological context of galaxy evolution, the study provides crucial insights into the transformative role that mergers and internal processes have played over cosmic time.