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Peer-reviewedCC BYNature Astronomy

Evidence of a massive accretion event 1.8 billion years before the Gaia-Sausage-Enceladus merger

  • D. Massari1
  • Chiara Zerbinati2,1
  • C. Fanelli1
  • A. Helmi3
  • E. Ceccarelli2,3,1
  • Fernando Aguado Agelet4,5
  • S. Cassisi6,7
  • Ewoud Wempe3,8
  • M. Monelli6,9
  • Andrea Bellini10
  • Thomas M. Callingham11,3
  • Hanneke C. Woudenberg3
6 more authors
  • Roger E. Cohen12
  • Carme Gallart13
  • E. Pancino14
  • Sara Saracino14
  • M. Salaris15,1
  • A. Mucciarelli2
15 affiliations
  1. 1Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, INAF, Bologna, Italy
  2. 2Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, Bologna, Italy
  3. 3Kapteyn Astronomical Institute, University of Groningen, Groningen, The Netherlands
  4. 4Escola de Enxeñaría de Telecomunicación, Universidade de Vigo, Vigo, Spain
  5. 5Universidad de La Laguna, La Laguna, Spain
  6. 6Osservatorio Astronomico di Abruzzo, INAF, Teramo, Italy
  7. 7Sezione di Pisa, INFN, Universitá di Pisa, Pisa, Italy
  8. 8LIRA, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université, Meudon, France
  9. 9Osservatorio Astronomico di Roma, INAF, Monte Porzio Catone, Italy
  10. 10Space Telescope Science Institute, Baltimore, MD, USA
  11. 11Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh, UK
  12. 12Department of Physics and Astronomy, Rutgers the State University of New Jersey, Piscataway, NJ, USA
  13. 13Instituto de Astrofísica de Canarias, La Laguna, Spain
  14. 14Osservatorio Astrofisico di Arcetri, INAF, Firenze, Italy
  15. 15Astrophysics Research Institute, Liverpool John Moores University, Liverpool, UK
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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.

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Cite

BibTeXRIS
BibTeX
@article{massari2026evidence,
  title = {{Evidence of a massive accretion event 1.8 billion years before the Gaia-Sausage-Enceladus merger}},
  author = {D. Massari and Chiara Zerbinati and C. Fanelli and A. Helmi and E. Ceccarelli and Fernando Aguado Agelet and S. Cassisi and Ewoud Wempe and M. Monelli and Andrea Bellini and Thomas M. Callingham and Hanneke C. Woudenberg and Roger E. Cohen and Carme Gallart and E. Pancino and Sara Saracino and M. Salaris and A. Mucciarelli},
  journal = {Nature Astronomy},
  year = {2026},
  month = aug,
  doi = {10.1038/s41550-026-02931-5},
  url = {https://doi.org/10.1038/s41550-026-02931-5},
  note = {CC BY},
}
RIS
TY  - JOUR
TI  - Evidence of a massive accretion event 1.8 billion years before the Gaia-Sausage-Enceladus merger
AU  - D. Massari
AU  - Chiara Zerbinati
AU  - C. Fanelli
AU  - A. Helmi
AU  - E. Ceccarelli
AU  - Fernando Aguado Agelet
AU  - S. Cassisi
AU  - Ewoud Wempe
AU  - M. Monelli
AU  - Andrea Bellini
AU  - Thomas M. Callingham
AU  - Hanneke C. Woudenberg
AU  - Roger E. Cohen
AU  - Carme Gallart
AU  - E. Pancino
AU  - Sara Saracino
AU  - M. Salaris
AU  - A. Mucciarelli
PY  - 2026
DA  - 2026/08/17
JO  - Nature Astronomy
DO  - 10.1038/s41550-026-02931-5
UR  - https://doi.org/10.1038/s41550-026-02931-5
AB  - 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.
SN  - 2397-3366
ER  -