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arXiv · 2610.30659

The Milky Way's Early Disk Was Built in Bursts. II. The Multi-component Age-Metallicity Structure of the High-$α$ Disk

Tiancheng Sun·Maosheng Xiang·Jifeng Liu·Meng Zhang·Yaqian Wu
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Abstract

Whether the high-$α$ disk experienced a continuous or episodic star-formation history remains an open question. Here we investigate the age--metallicity distribution of the high-$α$ disk using approximately 145,000 Gaia DR3 subgiants with $[\mathrm{Fe/H}]>-1$, 6.5~kpc $\lesssim R\lesssim9.5$~kpc, and $|Z|\lesssim1.5$~kpc. We adopt a forward-modelling framework that accounts for individual age and metallicity uncertainties and corrects for sample selection effects. The reconstructed distribution reveals two components: a broad, dominant population peaking at $\sim10.6$ Gyr and $[\mathrm{Fe/H}]\simeq-0.38$, and an older, more metal-poor population peaking at $\sim12.4$ Gyr and $[\mathrm{Fe/H}]\simeq-0.79$. These features suggest an episodic history of star formation and chemical enrichment in the high-$α$ disk, qualitatively consistent with cosmological simulations in which recurrent gas accretion drives variations in the star formation rate. The age of the older component is close to the $\sim12.3$~Gyr epoch inferred for the proposed Low-energy--Kraken--Heracles accretion event, suggesting a possible connection between this early interaction and enhanced disk star formation. There is tentative evidence that the dominant population is possibly composed of two sub-components with age distributions peaking at $\sim11.1$ Gyr and $\sim10.0$ Gyr, respectively. Together, these results favour a picture in which the high-$α$ disk was built through recurrent episodes of enhanced star formation rather than a single temporally smooth phase, even for the regime of ${\rm [Fe/H]}>-1$.

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Tiancheng Sun, Maosheng Xiang, Jifeng Liu, Meng Zhang, Yaqian Wu. 2026-10-22. The Milky Way's Early Disk Was Built in Bursts. II. The Multi-component Age-Metallicity Structure of the High-$α$ Disk. https://arxiv.org/abs/2610.30659

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