Stellar Age Compression Reshapes Interpretations of the Milky Way Thick-Disk Formation History

2026-05-11

AI Summary

This study demonstrates that apparent evidence for the rapid formation of the Milky Way’s thick disk may arise from a systematic compression bias in stellar age estimates. By comparing astroNN spectroscopic ages with APOKASC-3 asteroseismic ages and employing covariate matching alongside optimal transport inversion experiments, the authors show for the first time that age compression alone can reproduce observational signatures resembling burst-like formation, without requiring an actual rapid assembly history. After correcting for this bias, the thick disk exhibits a shallower age–metallicity relation, a significantly extended formation timescale, and a markedly delayed peak formation epoch. These findings weaken the existing case for rapid thick-disk formation and underscore the critical influence of stellar age definitions on inferences of galactic assembly histories.

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.

Problem

• stellar age compression
• Milky Way thick disk
• age-metallicity relation
• formation timescale
• Galactic archaeology

Innovation

• stellar age compression
• thick-disk formation
• age-metallicity relation
• asteroseismic ages
• transport map