On the origin of the chemical bimodality of disk stars: A tale of merger and migration

TL;DR

This study uses cosmological simulations to show that the bimodal alpha-element sequence in the Milky Way's disk results from a gas-rich merger, explaining the distinct properties and distribution of high- and low-alpha stars.

Contribution

It demonstrates that the alpha-bimodality naturally arises from a gas-rich merger event, linking chemical, kinematic, and spatial properties of disk stars in simulations.

Findings

Bimodal alpha-sequence is a common outcome of gas-rich mergers.

High-alpha stars are older, centrally concentrated, and have larger scale heights.

Low-alpha stars are younger, more extended, and formed after the merger.

Abstract

The Milky Way's stellar disk exhibits a bimodality in the [Fe/H] vs. [$\alpha$/Fe] plane, showing a distinct high-$\alpha$ and low-$\alpha$ sequence whose origin is still under debate. We examine the [Fe/H]-[$\alpha$/Fe] abundance plane in cosmological hydrodynamical simulations of Milky Way like galaxies from the NIHAO-UHD project and show that the bimodal $\alpha$-sequence is a generic consequence of a gas-rich merger at some time in the Galaxy's evolution. The high-$\alpha$ sequence evolves first in the early galaxies, extending to high metallicities, while it is the low-$\alpha$ sequence that is formed after the gas-rich merger. The merger brings in fresh metal-poor gas diluting the interstellar medium's metallicity while keeping the [$\alpha$/Fe] abundance almost unchanged. The kinematic, structural and spatial properties of the bimodal $\alpha$-sequence in our simulations reproduces that of observations. In all simulations, the high-$\alpha$ disk is old, radially concentrated towards the galaxy's center and shows large scale heights. In contrast, the low-$\alpha$ disk is younger, more radially extended and concentrated to the disk mid-plane. Our results show that the abundance plane is well described by these two populations that have been distributed radially across the disk by migration: at present-day in the solar neighbourhood, low-$\alpha$ stars originate from both the inner and outer disk while most of the high-$\alpha$ stars have migrated from the inner disk. We show that age dating the stars in the [Fe/H]-[$\alpha$/Fe] plane can constrain the time of the low-$\alpha$ sequence forming merger and conclude that $\alpha$-bimodality is likely a not uncommon feature of disk galaxies.