Portrait of a Galaxy on FIRE: Exploring α-bimodality as a consequence of inside-out disc growth in a hierarchical formation scenario

¹ Tartu Observatory, University of Tartu, Observatooriumi 1, Tõravere 61602, Estonia
² Instituto de Astrofísica de Canarias, Calle Vía Láctea s/n, 38206 La Laguna, Santa Cruz de Tenerife, Spain
³ Universidad de La Laguna, Avda. Astrofísico Francisco Sánchez 38205 La Laguna, Santa Cruz de Tenerife, Spain
⁴ Tallinn University of Technology, Ehitajate tee 5, Tallinn 19086, Estonia
⁵ National Institute of Chemical Physics and Biophysics (NICPB), Rävala 10, Tallinn 10143, Estonia

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 11 October 2025
Accepted: 21 March 2026

Abstract

Context. The chemical dichotomy in the [α/Fe]-[Fe/H] plane, such as the one observed in the stellar Milky Way disc, is a consequence of the complex processes underlying the formation and evolution of disc galaxies.

Aims. We determine the potential drivers behind the α-bimodality of the disc in a zoom-in hydrodynamical simulated galaxy with no prior major mergers and negligible radial migration.

Methods. Using a Milky Way-mass galaxy from the FIRE-2 suite of simulations, we analysed gas flows in the disc, together with its star formation and merger history, as well as the chemical evolution of the hot corona. These data allowed us to investigate their links to transitions in the chemo-dynamical structure of the stellar disc and its radial distribution.

Results. The simulated galaxy exhibits high and low-α sequences, without having experienced major mergers, nor significant radial migration in the past. A high-α 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. Next, accreted gas onto the disc from minor mergers mildly enhances the star formation rate and generates the low-α sequence in the outer disc, with radial inward flows of this material feeding the low-α 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. This serves as a further indication that the assumption of instantaneous gas mixing is invalid.

Conclusions. Understanding the chemical evolution of stellar discs calls for their accretion merger history and interaction with the surrounding hot corona to be accounted for, as well as the vertical and radial gas flows that redistribute metals within the disc.