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 ∼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.
Versions
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BibTeX
@misc{sun2026milky,
title = {{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}},
author = {Tiancheng Sun and Maosheng Xiang and Shaolan Bi and Jifeng Liu and Meng Zhang and Yaqian Wu},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.26374},
eprint = {2610.26374},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.26374},
}
RIS
TY - UNPB
TI - 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
AU - Tiancheng Sun
AU - Maosheng Xiang
AU - Shaolan Bi
AU - Jifeng Liu
AU - Meng Zhang
AU - Yaqian Wu
PY - 2026
DA - 2026/10/19
PB - arXiv
DO - 10.48550/arxiv.2610.26374
UR - https://arxiv.org/abs/2610.26374
AB - 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 ∼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.
ER -