On the origin of the chemical bimodality of disc stars: a tale of merger and migration

The Milky Way’s (MW) stellar disc exhibits a bimodality in the [Fe/H] versus [α/Fe] plane, showing a distinct high- and low-α sequences whose origin is still under debate. We examine the [Fe/H]–[α/Fe] abundance plane in cosmological hydrodynamical simulations of MW-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 disc is old, radially concentrated towards the galaxy’s centre and shows large scale heights. In contrast, the low-α disc is younger, more radially extended and concentrated to the disc mid-plane. Our results show that the abundance plane is well described by these two populations that have been distributed radially across the disc by migration: at present day in the solar neighbourhood, low-α stars originate from both the inner and outer discs, while most of the high-α stars have migrated from the inner disc. 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 disc galaxies.