Evidence of a massive accretion event 1.8 billion years before the Gaia-Sausage-Enceladus merger
Abstract The merger history of the Galaxy has been traced back firmly to a redshift of 2 (10 billion years ago). While evidence for the existence of at least one more substantial merger before this time has been presented, its interpretation is yet to be fully established. Here we show that the population of globular clusters around the Galaxy depicts three distinct age–metallicity sequences, one associated with the progenitor of the Milky Way, one with the merger with Gaia-Sausage-Enceladus 10 billion years ago and a third intermediate sequence associated with at least one merger that we estimate took place about 1.8 billion years before Gaia-Sausage-Enceladus. This discovery has been possible thanks to exquisite Hubble Space Telescope data and sophisticated analysis that enables very precise relative age determination of globular clusters. The newly identified sequence reveals that this merger took place with an object of a similar stellar mass to that of Gaia-Sausage-Enceladus (≃5 × 10 8 stellar masses, M ⊙ ) that deposited most of its mass in the inner 6 kpc of the Milky Way. The identification of a third merger event in the inner Galaxy puts to rest earlier debates and, honouring previous works, we name the progenitor system Low-energy-Kraken-Heracles, or LKH for short.
Authors
- M. Monelli (ORCID: https://orcid.org/0000-0001-5292-6380)
- M. Salaris (ORCID: https://orcid.org/0000-0002-2744-1928)
- E. Pancino (ORCID: https://orcid.org/0000-0003-0788-5879)
- D. Massari (ORCID: https://orcid.org/0000-0001-8892-4301)
- Ewoud Wempe (ORCID: https://orcid.org/0000-0001-8232-4188)
- S. Cassisi (ORCID: https://orcid.org/0000-0001-5870-3735)
- Fernando Aguado Agelet (ORCID: https://orcid.org/0000-0003-2713-1943)
- Hanneke C. Woudenberg (ORCID: https://orcid.org/0000-0003-4314-5632)
- A. Mucciarelli (ORCID: https://orcid.org/0000-0001-9158-8580)
- Thomas M. Callingham (ORCID: https://orcid.org/0000-0003-4952-1299)
- C. Fanelli (ORCID: https://orcid.org/0000-0002-4639-1364)
- Andrea Bellini (ORCID: https://orcid.org/0000-0003-3858-637X)
- E. Ceccarelli (ORCID: https://orcid.org/0009-0007-3793-9766)
- C. Zerbinati (ORCID: https://orcid.org/0009-0004-2797-4056)
- Helmi A (ORCID: https://orcid.org/0000-0003-3937-7641)
- Carme Gallart
- Sara Saracino
- Roger E. Cohen
Institutions
- Rutgers, The State University of New Jersey (US)
- Space Telescope Science Institute (US)
- Universidad de La Laguna (ES)
- University of Groningen (NL)
- Université Paris Cité (FR)
- Arcetri Astrophysical Observatory (IT)
- Astronomical Observatory of Rome (IT)
- Sorbonne Université (FR)
- Istituto di Astrofisica Spaziale e Fisica Cosmica di Bologna (IT)
- Sorbonne Paris Cité (FR)
- Royal Observatory (GB)
- Collurania Teramo Observatory (IT)
- Instituto de Astrofísica de Canarias (ES)
- Istituto Nazionale di Fisica Nucleare, Sezione di Pisa (IT)
- Osservatorio di Astrofisica e Scienza dello Spazio
- Universidade de Vigo (ES)
- Liverpool John Moores University (GB)
- University of Edinburgh (GB)
Publication Details
- Journal
- Nature Astronomy
- Published
- 2026-08-17
- DOI
- https://doi.org/10.1038/s41550-026-02931-5
- Citations
- 1
- Primary Topic
- Gamma-ray bursts and supernovae
- Type
- article
- Field-Weighted Citation Impact
- 4.71
Funders
- National Aeronautics and Space Administration
- Space Telescope Science Institute
- European Space Agency
- European Commission
- Nederlandse Organisatie voor Wetenschappelijk Onderzoek
- Ministero dell’Istruzione, dell’Università e della Ricerca
- Ministerio de Ciencia e Innovación
- Istituto Nazionale di Astrofisica
- European Regional Development Fund
- Agencia Estatal de Investigación