VINTERGATAN III: how to reset the metallicity of the Milky Way

TL;DR

This paper presents a simulation-based scenario where the Milky Way's low-metallicity stars form in an inclined outer disk triggered by a major merger, challenging traditional sequential formation models.

Contribution

It introduces a new in situ formation scenario for low-[$eta$/Fe] stars involving a tilting outer disk caused by a major merger, supported by cosmological simulations.

Findings

Outer disk star formation occurs at lower metallicity due to tidal compression.

The inclined outer disk aligns with the inner galaxy through gravitational torques.

The low-[$eta$/Fe] sequence forms via two distinct channels in the galaxy's history.

Abstract

Using the cosmological zoom simulation VINTERGATAN, we present a new scenario for the onset of star formation at the metal-poor end of the low-[$\alpha$/Fe] sequence in a Milky Way-like galaxy. In this scenario, the galaxy is fueled by two distinct gas flows. One is enriched by outflows from massive galaxies, but not the other. While the former feeds the inner galactic region, the latter fuels an outer gas disk, inclined with respect to the main galactic plane, and with a significantly poorer chemical content. The first passage of the last major merger galaxy triggers tidal compression in the outer disk, which increases the gas density and eventually leads to star formation, at a metallicity 0.75 dex lower than the inner galaxy. This forms the first stars of the low-[$\alpha$/Fe] sequence. These in situ stars have halo-like kinematics, similarly to what is observed in the Milky Way, due to the inclination of the outer disk which eventually aligns with the inner one via gravitational torques. We show that this tilting disk scenario is likely to be common in Milky-Way like galaxies. This process implies that the low-[$\alpha$/Fe] sequence is populated in situ, simultaneously from two formation channels, in the inner and the outer galaxy, with distinct metallicities. This contrasts with purely sequential scenarios for the assembly of the Milky Way disk and could be tested observationally.