Chronology of our Galaxy from Gaia colour-magnitude diagram fitting (ChronoGal)

III. Age and metallicity distribution of Gaia-Sausage-Enceladus stars near the Sun

¹ Universidad de Granada, Departamento de Física Teórica y del Cosmos, Campus Fuente Nueva, Edificio Mecenas, 18071 Granada, Spain
² Instituto Carlos I de Física Teórica y Computacional, Facultad de Ciencias, 18071 Granada, Spain
³ Instituto de Astrofísica de Canarias, Calle Vía Láctea s/n, 38206 La Laguna, Tenerife, Spain
⁴ Departamento de Astrofísica, Universidad de La Laguna, 38205 La Laguna, Tenerife, Spain
⁵ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
⁶ Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, Via Piero Gobetti 93/2, 40129 Bologna, Italy
⁷ Kapteyn Astronomical Institute, University of Groningen, Landleven 12, 9747 AD Groningen, The Netherlands
⁸ INAF – Astronomical Observatory of Abruzzo, via M. Maggini, sn, 64100 Teramo, Italy
⁹ INFN, Sezione di Pisa, Largo Pontecorvo 3, 56127 Pisa, Italy
¹⁰ atlanTTic, Universidade de Vigo, Escola de Enxeñaría de Telecomunicación, 36310 Vigo, Spain
¹¹ Universidad de La Laguna, Avda. Astrofísico Fco. Sánchez, 38205 La Laguna, Tenerife, Spain
¹² INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone Roma, Italy
¹³ Institute for Computational Cosmology, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK

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

Received: 26 February 2025
Accepted: 21 May 2025

Abstract

Context. Gaia-Sausage-Enceladus is considered the last major merger that contributed to the formation of the Milky Way. Its remnants dominate the nearby accreted stellar halo of the Milky Way.

Aims. We aim to characterise the star formation history of Gaia-Sausage-Enceladus through the age and metallicity of its stellar populations.

Methods. From Gaia DR3 data, we dynamically defined three Gaia-Sausage-Enceladus samples with different criteria and possible degrees of contamination from other substructures in the halo. Then, we derived the stellar age and metallicity distributions using the CMDft.Gaia package.

Results. We identified three main populations of stars and a fourth smaller one following an almost linear age-[M/H] relation. We associate the three oldest populations with the bulk of the star formation that lasted for, at least, ∼ 3−4 Gyr and ended about 10 Gyr ago, its metallicities ranging from −1.7 to −0.8. We categorise these populations into two main epochs: the evolution of GSE in isolation and the merger event. This separation finds independent support from the age-metallicity relation of GSE globular clusters. The fourth population is younger and more metal-rich, at ∼ 8.5 Gyr and [M/H] ∼−0.4; its link to GSE is unclear.