Stellar Age Compression Reshapes Interpretations of the Milky Way Thick-Disk Formation History
2026-05-11 • Machine Learning
Machine Learning
AI summaryⓘ
The authors investigate how different methods of estimating star ages affect our understanding of how quickly the Milky Way's thick disk formed. They compare ages derived from spectroscopy and asteroseismology and find that previous ideas of a fast thick disk formation become less clear when using seismic ages. Their analysis shows that some observed patterns supporting rapid formation could actually result from how ages are measured and compressed, not from the galaxy's true history. This suggests that conclusions about the Milky Way's formation timescale may change depending on which age estimates are used.
Milky Way thick diskage-metallicity relation (AMR)stellar agesspectroscopic agesasteroseismic agesGalactic archaeologychemical evolutionformation timescaleastroNNAPOKASC-3
Authors
Zhipeng Zhang
Abstract
The formation timescale of the Milky Way thick disk is one of the central debates in Galactic archaeology. The age-metallicity relation (AMR), formation timescale, and chemical evolution gradients are frequently used to infer a rapid assembly, short-timescale enrichment, and bursty formation history of the thick disk. However, stellar ages are not directly observable, introducing the potential risk that inferred ages may harbor a systematic compression tied to observational quality. In this paper, we use the same stellar sample and identical physical covariate matching conditions, but two independent age scales--spectroscopic inferred ages (astroNN) and asteroseismic ages (APOKASC-3)--to compare the observable signatures of the thick-disk formation history. We find that several key observables previously supporting a rapid thick-disk formation are systematically weakened under seismic anchoring: the AMR slope flattens from -3.29 to -1.86 Gyr dex-1 (Delta a = +1.43), the formation timescale widens from 3.04 to 3.55 Gyr, and the peak formation age shifts from 9.1 to 6.0 Gyr. Through transport inversion experiments, we further show that additive noise can only broaden the age distribution and cannot reproduce the above pattern, whereas a compressive transport map (lambda < 1) simultaneously reproduces a narrower age distribution, a steeper AMR, and rapid-formation-like observables. This result indicates that the compression transformation itself is sufficient to generate rapid-formation-friendly observables without requiring an intrinsically bursty formation history. Our findings reveal that statistical interpretations of the Milky Way formation history may depend sensitively on the stellar age definition itself.