Age-Dating Red Giant Stars Associated with Galactic Disk and Halo Substructures

TL;DR

This study estimates stellar ages for Milky Way halo and disk stars using combined kinematic, asteroseismic, and spectroscopic data, providing insights into the Galaxy's merger history and substructure formation.

Contribution

It introduces a robust framework for inferring ages of galactic substructure stars, improving age estimates by combining multiple data sources and applying multiple age determination methods.

Findings

Gaia-Enceladus-Sausage stars have an average age of 8 ± 3 Gyr.

Hierarchical ages for halo and disk populations are around 8 Gyr.

The method demonstrates robustness despite systematic differences in age estimates.

Abstract

The vast majority of Milky Way stellar halo stars were likely accreted from a small number ($\lesssim$3) of relatively large dwarf galaxy accretion events. However, the timing of these events is poorly constrained, relying predominantly on indirect dynamical mixing arguments or imprecise age measurements of stars associated with debris structures. Here, we aim to infer robust stellar ages for stars associated with galactic substructures to more directly constrain the merger history of the Galaxy. By combining kinematic, asteroseismic, and spectroscopic data where available, we infer stellar ages for a sample of 10 red giant stars that were kinematically selected to be associated with the stellar halo, a subset of which are associated with the Gaia-Enceladus-Sausage halo substructure, and compare their ages to 3 red giant stars in the Galactic disk. Despite systematic differences in both absolute and relative ages determined by this work, age rankings of stars in this sample are robust. Passing the same observable inputs to multiple stellar age determination packages, we measure a weighted average age for the Gaia-Enceladus-Sausage stars in our sample of 8 $\pm$ 3 (stat.) $\pm$ 1 (sys.) Gyr. We also determine hierarchical ages for the populations of Gaia-Enceladus-Sausage, in situ halo and disk stars, finding a Gaia-Enceladus-Sausage population age of 8.0$^{+3.2}_{-2.3}$ Gyr. Although we cannot distinguish hierarchical population ages of halo or disk structures with our limited data and sample of stars, this framework should allow distinct characterization of Galactic substructures using larger stellar samples and additional data available in the near future.