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

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)