Rising from the Ashes: A Metallicity-dependent Star Formation Gap Splits the Milky Way’s α Sequences

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 [α/Fe]–[Fe/H], it is now clear that two separate populations exist—the low-α and high-α sequences. We propose that a brief (∼300 Myr) halt in star formation within a narrow metallicity bin can lead to a bimodal [α/Fe] distribution at that metallicity, assuming a rapidly declining gas phase [α/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 nonmerger scenarios. We predict a ∼300 Myr gap in stellar ages at a fixed [Fe/H] where the α bimodality is prominent ([Fe/H] ≲ −0.2).