Rising from the Ashes. II. The Bar-driven Abundance Bimodality of the Milky Way

Abstract

The Milky Way hosts at least two modes in its present-day distribution of Fe and α -elements. The exact cause of this bimodality is disputed, but one class of explanations involves the merger between the Milky Way and a relatively massive satellite (Gaia-Sausage-Enceladus) at z ∼ 2. However, reproducing this bimodality in simulations is not straightforward, with conflicting results on the prevalence, morphology, and mechanism behind multimodality. We present a case study of a galaxy in the Illustris TNG50 simulation that undergoes sequential phases of starburst, brief quiescence, and then rejuvenation. This scenario results in a pronounced abundance bimodality after a post-processing adjustment of the [ α /Fe] of old stars designed to mimic a higher star formation efficiency in dense gas. The high- and low- α sequences are separated in time by the brief quiescent period, which is associated not with a merger but with the formation of a bar followed by active galactic nucleus (AGN) activity. This galaxy indicates a novel scenario in which the α -bimodality in the Milky Way is caused by the formation of the bar via AGN-induced quenching. In addition to a stellar age gap in the Milky Way, we predict that abundance bimodalities should be more common in barred as opposed to unbarred galaxies.

Keywords

• the Milky Way
• Galactic bar
• Active galactic nuclei
• Post-starburst galaxies
• Magnetohydrodynamical simulations