TOPoS

VII. Age−metallicity relation in the Galactic halo and assembly of the Milky Way

¹ LIRA, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université, CNRS, 92190 Meudon, France
² Univ Rennes, CNRS, IPR (Institut de Physique de Rennes) – UMR 6251, 35000 Rennes, France
³ Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, Via G. Sansone 1, 50019, Sesto Fiorentino, Italy
⁴ Department of Physics and Astronomy, University of Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
⁵ INAF, Astrophysics and Space Science Observatory, Via Gobetti 93/3, 40129 Bologna, Italy
⁶ European Southern Observatory, Casilla 19001, Santiago, Chile
⁷ Universidad Andres Bello, Facultad de Ciencias Exactas, Departamento de Física y Astronomía – Instituto de Astrofísica, Autopista Concepción-Talcahuano 7100, Talcahuano, Chile
⁸ LIRA, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université, CNRS, 75014 Paris, France
⁹ UPJV, Université de Picardie Jules Verne, 33 rue St Leu, 80080 Amiens, France
¹⁰ INAF/Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy

^★ Corresponding author.

Received: 3 April 2026
Accepted: 8 June 2026

Abstract

Context. One technique for determining stellar ages is to compare the position of a star in the Hertzsprung-Russell diagram to theoretical stellar evolutionary tracks. The sub-giant evolutionary stage is the one that is most sensitive to age and allows the most precise evolutionary age estimates.

Aims. The TOPoS sample of stars with metallicities derived from low-resolution Sloan Digital Sky Survey spectra contains a large subset of sub-giant stars with precise parallaxes from the Gaia mission, for which evolutionary ages can be determined. Our aim is to use this stellar sample to investigate the age-metallicity relation in the Galactic halo.

Methods. We use the Bayesian inference code SPInS and theoretical BaSTI stellar evolutionary tracks to determine the ages for TOPoS stars.

Results. There is a clear increase in metallicity with decreasing age, albeit with a considerable scatter at any given age. At ages larger than 8 Ga, the scatter is so large that in fact, over this range, age and metallicity appear to be uncorrelated. At any given age, the metallicity distribution is multi-modal, with up to three distinct peaks. These peaks trace three age-metallicity relations that we tentatively identify with the halo, thick-disc, and thin-disc.

Conclusions. Our data demonstrate the important role of mergers in the evolution of the Galaxy, up to 8 Ga ago. In more recent times, the spread in metallicity drops. One possibility is that the major merger Gaia-Sausage-Enceladus may have perturbed the galaxies in the Milky Way vicinity in such a way as to decrease the merger rate. Chemical evolution models and cosmological models of the Local Group both support the importance of mergers in the early evolution of the Milky Way. Larger, unbiased samples, or at least with well-understood biases, of stars with accurate ages are required for a quantitative comparison between models and data.