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Peer-reviewedCC BY 4.0The Astronomical Journal

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

  • Tiancheng Sun1
  • Maosheng Xiang1,2
  • Shaolan Bi2
  • Jifeng Liu1,2,3
  • Meng Zhang1
  • Yaqian Wu1
  1. 1Chinese Academy of Sciences
  2. 2Beijing Normal University
  3. 3University of Chinese Academy of Science
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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.

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Cite

BibTeXRIS
BibTeX
@article{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},
  journal = {The Astronomical Journal},
  year = {2026},
  month = oct,
  doi = {10.3847/1538-3881/aeac24},
  url = {https://iopscience.iop.org/article/10.3847/1538-3881/aeac24},
  note = {CC BY 4.0},
}
RIS
TY  - JOUR
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  - 2026/10/16
JO  - The Astronomical Journal
DO  - 10.3847/1538-3881/aeac24
UR  - https://iopscience.iop.org/article/10.3847/1538-3881/aeac24
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 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.
SN  - 0004-6256
SN  - 1538-3881
ER  -