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

Abstract

Whether the high-$\alpha$ disk experienced a continuous or episodic star-formation history remains an open question. Here we investigate the age--metallicity distribution of the high-$\alpha$ disk using approximately 145,000 Gaia DR3 subgiants with $[Fe/H]>-1$, 6.5~kpc $\lesssim R\lesssim 9.5$~kpc, and $|Z|\lesssim 1.5$~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 $\sim 10.6$ Gyr and $[Fe/H]\simeq -0.38$, and an older, more metal-poor population peaking at $\sim 12.4$ Gyr and $[Fe/H]\simeq -0.79$. These features suggest an episodic history of star formation and chemical enrichment in the high-$\alpha$ 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 $\sim 12.3$~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 $\sim 11.1$ Gyr and $\sim 10.0$ Gyr, respectively. Together, these results favour a picture in which the high-$\alpha$ disk was built through recurrent episodes of enhanced star formation rather than a single temporally smooth phase, even for the regime of $[Fe/H]>-1$.

arXiv comment19 pages, 13 figures, Accepted for publication in ApJ