Portrait of a Galaxy on FIRE: Is the $\alpha$-bimodality a natural consequence of inside-out disc growth in a hierarchical formation scenario?

Abstract

The chemical dichotomy in the [$\alpha$/Fe]-[Fe/H] plane is a consequence of the complex processes underlying the formation and evolution of disc galaxies such as observed in the stellar Milky Way disc. We determine what can drive an $\alpha$-bimodality of the disc in a zoom-in hydrodynamical simulated galaxy which has had no major mergers and negligible radial migration. Using a Milky Way-mass galaxy from the FIRE-2 suite of simulations, we analyse gas flows in the disc together with its star formation and merger history, as well as the chemical evolution of the hot corona, to investigate their connection to transitions in the chemo-dynamical structure of the stellar disc and its radial distribution. The simulated galaxy exhibits high and low-$\alpha$ sequences without having experienced major mergers nor significant radial migration. A high-$\alpha$ thick disc forms during the early chaotic clustering phase. Afterwards, as the star formation rate declines, a dip in the stellar number density appears, coinciding with the dilution of the galactic corona by a minor merger, which subsequently halts the rise of [Fe/H] in the disc. Later, accreted gas onto the disc from minor mergers, mildly enhances the star formation rate and generates the low-$\alpha$ sequence in the outer disc, with radial inward flows of this material feeding the low-$\alpha$ inner disc. Furthermore, we find that even at fixed radii, newly formed stars retain a sizable spread in their chemical abundances, reflecting chemical differences between the in-situ and the infalling gas from which they formed, further indicating that instantaneous gas mixing is invalid. Understanding the chemical evolution of stellar discs requires accounting for their accretion merger history and interaction with the surrounding hot corona, as well as the vertical and radial gas flows that redistribute metals within the disc.