Chemo-dynamical reconstruction of Milky Way globular cluster progenitors: Age–metallicity relations and the universality of multiple stellar populations

¹ Dipartimento di Fisica e Astronomia “Augusto Righi”–Università di Bologna, via Piero Gobetti 93/2, 40129 Bologna, Italy
² INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, via Piero Gobetti 93/3, 40129 Bologna, Italy
³ Berkeley Center for Cosmological Physics and Department of Physics, University of California, Berkeley, CA 94720, USA
⁴ ICC, University of Barcelona, Martí i Franquès, 1, E08028 Barcelona, Spain
⁵ ICREA, Pg. Lluis Companys 23, Barcelona 08010, Spain

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 12 March 2026
Accepted: 17 April 2026

Abstract

Context. Globular clusters encode the hierarchical assembly history of the Milky Way and the internal physics of multiple stellar populations. Reconstructing progenitor-specific age–metallicity relations requires stellar parameters free from helium-driven age biases, yet whether multiple-population properties carry an environmental imprint remains an open question.

Aims. We reconstructed progenitor-specific age–metallicity relations for 69 Galactic globular clusters using homogeneous stellar parameters derived while modelling multiple stellar populations, and tested whether helium-related multiple-population properties depend on progenitor origin once cluster mass and metallicity are controlled for.

Methods. Ages, helium spreads (δY), mean helium abundances (Ȳ), and first-population fractions (f_P1) were drawn from hierarchical Bayesian colour–magnitude diagram modelling. Progenitor families were identified via probabilistic chemo-dynamical clustering, age–metallicity relations reconstructed within a hierarchical Bayesian framework, and multiple-population indicators tested for environmental dependence using regression models with sensitivity tests.

Results. Enrichment timescales are broadly consistent with τ ≲ 2 Gyr, though individual progenitors prefer shorter values when fitted independently and inter-progenitor differences cannot be resolved at the present sample size. The primary distinction is the extent of chemical evolution: most systems reach Δ[Fe/H] ≃ 1.1–1.3 dex, while Sagittarius achieves Δ[Fe/H] ≃ 1.6 dex and higher terminal metallicities. Progenitor masses identify Gaia–Sausage–Enceladus and low-energy group Nas the dominant events. Neither δY nor Ȳ shows a significant progenitor dependence, and the mass multiple-population scaling is indistinguishable across in situ and accreted systems; Sequoia clusters alone show higher f_P1 at fixed mass and metallicity.

Conclusions. Age–metallicity relations carry fossil signatures of the chemical evolution and mass hierarchy of progenitor galaxies. The extent of enrichment, but not its pace, distinguishes progenitor systems. Helium enrichment amplitude is primarily regulated by cluster mass and blind to environment, pointing to universal cluster-scale formation physics. The sole robust exception of a residual progenitor dependence is in f_P1, suggesting the enriched-star fraction retains a secondary environmental imprint.