The Milky Way’s Early Disk Was Built in Bursts. I. Evidence from Low-angular-momentum, High-α Stars Scattered Out of the Protodisk by Merger Events

Abstract

Stars born in the high-α disk can later acquire low angular momentum through dynamical scattering during early merger events. Their age–metallicity distribution thus provides a fossil record of the history of both star formation and dynamical perturbations in the early Milky Way disk. Here we identify a sample of 2078 such stars from more than two million Gaia Data Release 3 subgiants with robust determinations of ages, kinematics, and abundances. From this sample we reconstruct their intrinsic age–metallicity distribution with a forward modeling approach. Our results confirm that these scattered stars are older and more metal poor than the bulk of the high-α disk on average. In particular, the joint age–metallicity distribution reveals a robust two-component feature: a broad, dominant component with a peak age of ∼11.5 Gyr and [Fe/H] ≃ − 0.59, and an older and more metal-poor component with a peak age of ∼12.5 Gyr and [Fe/H] ≃ − 0.86. The presence of this two-component structure suggests that the high-α disk experienced episodic phases of star formation and chemical enrichment. The age distribution of the younger Splash component, compared with that of the high-α disk, provides evidence for a substantial dynamical perturbation at a lookback time of 10.8–11.5 Gyr, possibly associated with an early passage of Gaia–Sausage–Enceladus (GSE). The older Splash component, with a peak age of ∼12.5 Gyr, may preserve signatures of an earlier interaction, possibly associated with the proposed low-energy Kraken–Heracles (LKH) event.