Rising from the Ashes: A Metallicity-Dependent Star Formation Gap Splits the Milky Way's alpha-Sequences

TL;DR

This paper proposes that a brief star formation halt caused by a galaxy merger can create the observed bimodal alpha-element distribution in the Milky Way, linking chemical signatures to galaxy evolution events.

Contribution

It introduces a metallicity-dependent star formation gap as a mechanism for the alpha-sequence split, supported by idealized galaxy merger simulations.

Findings

A ~300 Myr star formation gap can produce bimodal [alpha/Fe] distributions.

The Gaia-Sausage-Enceladus merger at z~2 may have triggered this gap.

Predicted age gap of ~300 Myr at specific metallicities.

Abstract

The elemental abundance distribution of stars encodes the history of the gas-phase abundance in the Milky Way. Without a large, unbiased sample of highly precise stellar ages, the exact timing and nature of this history must be inferred from the abundances. In the two-dimensional plane of [alpha/Fe]-[Fe/H], it is now clear that two separate populations exist -- the low-alpha and high-alpha sequences. We propose that a brief (~300 Myr) halt in star formation within a narrow metallicity bin can lead to a bimodal [alpha/Fe] distribution at that metallicity, assuming a rapidly declining gas phase [alpha/Fe]. Using simulations of an idealized setup of a high-z galaxy merger, we show that the merger with the Gaia-Sausage-Enceladus satellite at z~2 is one possible way to trigger such a gap in the Milky Way. This mechanism may also operate in non-merger scenarios. We predict a ~300 Myr gap in stellar ages at a fixed [Fe/H] where the alpha-bimodality is prominent ([Fe/H]<~-0.2)