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

Abstract

The Milky Way's stellar disk exhibits a bimodality in the [Fe/H] vs. [$α$/Fe] plane, showing a distinct high-$α$ and low-$α$ sequence whose origin is still under debate. We examine the [Fe/H]-[$α$/Fe] abundance plane in cosmological hydrodynamical simulations of Milky Way like galaxies from the NIHAO-UHD project and show that the bimodal $α$-sequence is a generic consequence of a gas-rich merger at some time in the Galaxy's evolution. The high-$α$ sequence evolves first in the early galaxies, extending to high metallicities, while it is the low-$α$ 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 [$α$/Fe] abundance almost unchanged. The kinematic, structural and spatial properties of the bimodal $α$-sequence in our simulations reproduces that of observations. In all simulations, the high-$α$ disk is old, radially concentrated towards the galaxy's center and shows large scale heights. In contrast, the low-$α$ 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-$α$ stars originate from both the inner and outer disk while most of the high-$α$ stars have migrated from the inner disk. We show that age dating the stars in the [Fe/H]-[$α$/Fe] plane can constrain the time of the low-$α$ sequence forming merger and conclude that $α$-bimodality is likely a not uncommon feature of disk galaxies.