VINTERGATAN I: The origins of chemically, kinematically and structurally distinct discs in a simulated Milky Way-mass galaxy (2006.06008v2)

Abstract: Spectroscopic surveys of the Milky Way's stars have revealed spatial, chemical and kinematical structures that encode its history. In this work, we study their origins using a cosmological zoom simulation, VINTERGATAN, of a Milky Way-mass disc galaxy. We find that in connection to the last major merger at $z\sim 1.5$, cosmological accretion leads to the rapid formation of an outer, metal-poor, low-[$\alpha$/Fe] gas disc around the inner, metal-rich galaxy containing the old high-[$\alpha$/Fe] stars. This event leads to a bimodality in [$\alpha$/Fe] over a range of [Fe/H]. A detailed analysis of how the galaxy evolves since $z\sim 1$ is presented. We demonstrate the way in which inside-out growth shapes the radial surface density and metallicity profile and how radial migration preferentially relocates stars from the inner to the outer disc. Secular disc heating is found to give rise to increasing velocity dispersions and scaleheights with stellar age, which together with disc flaring explains several trends observed in the Milky Way, including shallower radial [Fe/H]-profiles above the midplane. We show how the galaxy formation scenario imprints non-trivial mappings between structural associations (i.e. thick and thin discs), velocity dispersions, $\alpha$-enhancements, and ages of stars, e.g. the most metal-poor stars in the low-[$\alpha$/Fe] sequence are found to have a scaleheight comparable to old high-[$\alpha$/Fe] stars. Finally, we illustrate how at low spatial resolution, comparable to the thickness of the galaxy, the proposed pathway to distinct sequences in [$\alpha$/Fe]-[Fe/H] cannot be captured.

• The paper identifies a chemical bimodality driven by a major merger at z ~1.5, leading to the formation of an outer low-metallicity disc.
• It demonstrates that internal disc heating and radial migration shape the kinematic and structural evolution of stellar components.
• The study highlights an inside-out growth pattern, refining our theoretical framework for galactic disc evolution in cosmological contexts.

Insights into the Structure and Formation of the Milky Way-Mass Galaxy in the VINTERGATAN Simulation

The paper "VINTERGATAN I: The origins of chemically, kinematically and structurally distinct discs in a simulated Milky Way-mass galaxy" by Oscar Agertz et al. offers a detailed computational paper of the formation and evolution of a Milky Way-mass galaxy using a high-resolution cosmological zoom-in simulation known as VINTERGATAN. This research provides valuable insights into the formation mechanisms of distinct stellar disc components within a galaxy of this scale, emphasizing the influence of cosmic accretion, star formation, and internal movement of stellar material over time.

Formation and Evolution Characteristics

The paper focuses on the development of spatial, chemical, and kinematical structures within the simulated galaxy, particularly addressing the origins of chemically bimodal features observed in the Milky Way. The authors utilize the VINTERGATAN simulation to test theories of galaxy formation, complement observational studies and provide a framework for understanding the evolution of galactic disc structures.

1. Chemical Bimodality: The paper identifies a significant chemical bimodality characterized by two distinct sequences in the - plane. This emergence is linked to the last major merger event at a redshift of approximately z ~ 1.5, where cosmological accretion precipitated the rapid formation of an outer, low-metallicity, low- star-forming gas disc surrounding the older high-metallicity disc. This outer disc formation is highlighted as a primary driver for the observed separation in alpha-element enhancement across a range of metallicities.
2. Kinematic and Structural Properties: The authors demonstrate the dynamic interplay between the kinematic and structural components of galactic discs. The simulation reveals that secular disc heating contributes to increasing velocity dispersions and disc scaleheights as the stellar population ages, a feature that also exhibits radial migration from inner to outer regions.
3. Inside-Out Growth: The VINTERGATAN simulation underscores the galaxy's inside-out growth pattern, shaping both the radial surface density and metallicity profiles over time. The gradual alignment of initially misaligned stellar discs into a coherent structure further reflects on how internal processes reshape galactic architecture post major cosmic events.

Implications and Future Developments

The findings from this paper have implications for both theoretical understandings and observational approaches in astronomy. By offering a detailed model of how chemically, kinematically, and structurally distinct disc components arise and evolve, the paper enhances theoretical frameworks that can be used to interpret observations from current and future telescopic surveys. Furthermore, the recognition of complex mappings between structural associations, velocity dispersions, -enhancements, and stellar ages suggests potentially nuanced connections that extend beyond traditional thin-thick disc paradigms.

Future developments in astrophysical simulations and observational technologies could further refine these insights. The combined use of higher-resolution simulations along with data from advanced spectroscopic surveys could provide more comprehensive constraint mechanisms, allowing researchers to explore the more varied evolutionary pathways of galaxy formation. Additionally, variations in initial conditions, merger histories, and feedback processes remain rich areas for future inquiry, potentially unveiling how unique or common the modeled pathways are among disc galaxies similar to our own Milky Way.

This paper sets a precedent for the intricate and interconnected nature of galaxy formation processes, highlighting the importance of detailed simulations in uncovering the complexities inherent in galactic evolution and structure formation.