Chronologically dating the early assembly of the Milky Way

Josefina Montalbán, John Ted Mackereth, Andrea Miglio, Fiorenzo Vincenzo, Cristina Chiappini, Gaël Buldgen, Benoît Mosser, Arlette Noels, Richard Scuflaire, Mathieu Vrard, Emma Willett, Guy R. Davies, Oliver Hall, Martin Bo Nielsen, Saniya Khan, Ben M. Rendle, Walter E. van Rossem, Jason W. Ferguson, William J. Chaplin

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The standard cosmological model predicts that galaxies are built through hierarchical assembly on cosmological timescales1,2. The Milky Way, like other disk galaxies, underwent violent mergers and accretion of small satellite galaxies in its early history. Owing to Gaia Data Release 23 and spectroscopic surveys4, the stellar remnants of such mergers have been identified5,6,7. The chronological dating of such events is crucial to uncover the formation and evolution of the Galaxy at high redshift, but it has so far been challenging due to difficulties in obtaining precise ages for these oldest stars. Here we combine asteroseismology—the study of stellar oscillations—with kinematics and chemical abundances to estimate precise stellar ages (~11%) for a sample of stars observed by the Kepler space mission8. Crucially, this sample includes not only some of the oldest stars that were formed inside the Galaxy but also stars formed externally and subsequently accreted onto the Milky Way. Leveraging this resolution in age, we provide compelling evidence in favour of models in which the Galaxy had already formed a substantial population of its stars (which now reside mainly in its thick disk) before the infall of the satellite galaxy Gaia-Enceladus/Sausage5,6 around 10 billion years ago.
Original languageEnglish
Pages (from-to)640-647
JournalNature Astronomy
Issue number7
Early online date17 May 2021
Publication statusE-pub ahead of print - 17 May 2021

Bibliographical note

Updated with the final version accepted for publication as a Letter in Nature Astronomy (29 pages, 10 figures, 2 tables, including main article and methods section). Improved presentation of results, which have not changed from the previous version


  • astro-ph.GA
  • astro-ph.SR


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