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 2,078 such stars from more than two million Gaia DR3 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 $\sim$11.5~Gyr and ${\rm [Fe/H]}\simeq-0.59$, and an older and more metal-poor component with a peak age of $\sim$12.5~Gyr and ${\rm [Fe/H]}\simeq-0.86$. 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 $\sim$12.5~Gyr, may preserve signatures of an earlier interaction, possibly associated with the proposed Low-energy–Kraken–Heracles (LKH) event.