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Cited by 441Stellar, planetary, and galactic studiesGalaxies: Formation, Evolution, PhenomenaAstrophysics and Star Formation Studies

Origin of the system of globular clusters in the Milky Way

Open access
Massari, D., Koppelman, H. H., Helmi, A.·2019·Astronomy & Astrophysics
DOI 10.1051/0004-6361/201936135
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Abstract

Context . The assembly history experienced by the Milky Way is currently being unveiled thanks to the data provided by the Gaia mission. It is likely that the globular cluster system of our Galaxy has followed a similarly intricate formation path. Aims . To constrain this formation path, we explore the link between the globular clusters and the known merging events that the Milky Way has experienced. Methods . To this end, we combined the kinematic information provided by Gaia for almost all Galactic clusters, with the largest sample of cluster ages available after carefully correcting for systematic errors. To identify clusters with a common origin we analysed their dynamical properties, particularly in the space of integrals of motion. Results . We find that about 40% of the clusters likely formed in situ. A similarly large fraction, 35%, appear to be possibly associated to known merger events, in particular to Gaia -Enceladus (19%), the Sagittarius dwarf galaxy (5%), the progenitor of the Helmi streams (6%), and to the Sequoia galaxy (5%), although some uncertainty remains due to the degree of overlap in their dynamical characteristics. Of the remaining clusters, 16% are tentatively associated to a group with high binding energy, while the rest are all on loosely bound orbits and likely have a more heterogeneous origin. The resulting age–metallicity relations are remarkably tight and differ in their detailed properties depending on the progenitor, providing further confidence on the associations made. Conclusions . We provide a table listing the likely associations. Improved kinematic data by future Gaia data releases and especially a larger, systematic error-free sample of cluster ages would help to further solidify our conclusions.

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  • The low-energy GCs identified by Massari et al. (2019) represent a significant fraction of the satellite branch in age-metallicity space and thus likely represent the Kraken progenitor event.

    “These GCs represent a significant fraction of the satellite branch in age-metallicity space and thus likely represent the Kraken progenitor event needed to explain the age-metallicity 'satellite branch' GCs after Sagittarius and Gaia -Enceladus have been accounted for.”

    From Kraken reveals itself – the merger history of the Milky Way reconstructed with the E-MOSAICS simulations (2020) · cites this paper as Massari et al. 2019

    Section 3.2.5 identifies 25 low-energy clusters as tentative members of a group not associated with known mergers. The Summary and conclusions says they have a coherent age–metallicity relation and may reflect another accretion event, such as a Kraken-like merger. It does not establish what fraction of the satellite branch they represent or identify them as members of the Kraken event; it explicitly says its proposed clusters differ from previously reported Kraken candidates. Page numbers are unavailable in our copy. (LLM, conf=0.86)

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  • The orbital properties of NGC6535 (R_a ~ 4.5 kpc and e ~ 0.63) clearly place it in a part of orbital parameter space not covered by any Sequoia globular clusters and are rather consistent with the Kraken globular clusters.

    “While the position of NGC6535 in age-metallicity space ([Fe = H] = 1 : 73 and = 12 : 2 0 : 6 Gyr) does not allow distinguishing between Kraken and Sequoia, its orbital properties ( R a 4 : 5 kpc and 0 : 63) clearly place it in a part of the orbital parameter space that is not covered by any of the Sequoia GCs (which occupy R a > 25 kpc in the selection of Massari et al. 2019) and is rather”

    From Kraken reveals itself – the merger history of the Milky Way reconstructed with the E-MOSAICS simulations (2020) · cites this paper as Massari et al. 2019

    Section 3.2.4 prioritises assigning NGC 6535 to the low-energy group, and the Summary and conclusions describe that group as Kraken-like. But Table A.1 retains a tentative Sequoia association for NGC 6535. The cited paper does not give the quoted apocentre and eccentricity for NGC 6535 or establish that all its selected Sequoia clusters have apocentres above 25 kpc, so it does not support the claim's categorical orbital separation. (LLM, conf=0.84)

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  • Massari et al. (2019) identify five groups of Galactic globular clusters that plausibly have an extragalactic origin.

    “Massari et al. (2019) combine the orbital information of the Galactic GC system with their age-metallicity distribution to identify subsets of GCs that likely originated in a common progenitor. By dividing the GC population in this way, they identify five groups of GCs that plausibly have an extragalactic origin.”

    From Kraken reveals itself – the merger history of the Milky Way reconstructed with the E-MOSAICS simulations (2020) · cites this paper as Massari et al. (2019)

  • NGC6121 is too metal-poor to have formed in-situ, based on its age (12.2 ± 0.5 Gyr) and metallicity ([Fe/H] = -1.14).

    “Based on the age and metallicity of NGC6121 ( = 12 : 2 0 : 5 Gyr and [Fe = H] = 1 : 14), we consider it too metal-poor to have formed in-situ and follow the choice of Massari et al. (2019) to associate it with the low-energy group, even if we acknowledge that this is an edge case.”

    From Kraken reveals itself – the merger history of the Milky Way reconstructed with the E-MOSAICS simulations (2020) · cites this paper as Massari et al. (2019)

  • Modulo a small number of changes, we adopt the proposed GC membership of the satellite progenitors from Massari et al. (2019).

    “Modulo a small number of changes, we adopt the proposed GC membership of the satellite progenitors from Massari et al. (2019, see our Table 1).”

    From Kraken reveals itself – the merger history of the Milky Way reconstructed with the E-MOSAICS simulations (2020) · cites this paper as Massari et al. (2019)

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Sorted by citations

Authors
Massari, D., Koppelman, H. H., & Helmi, A.
Affiliations
University of Groningen (NL)
ORCID
Venue
Astronomy & Astrophysics
Publisher
EDP Sciences
Published
2019-09-23
Volume
630
Pages
L4
ISSN
  • 0004-6361 (ISSN-L)
  • 1432-0746
DOI
10.1051/0004-6361/201936135
OpenAlex
W2972524109
Cited by
441
References
50
Citations by year
2026: 48 · 2025: 75 · 2024: 54 · 2023: 58 · 2022: 56 · 2021: 80 · 2020: 59 · 2019: 10 · 2017: 1
FWCI
27.36
Type
journal article
Language
en
Open access
Licence
www.edpsciences.org/en/authors/copyright-and-licensing (vor)
Locations
  • Astronomy and Astrophysics (publishedVersion)
  • arXiv (Cornell University) (submittedVersion)
  • Archivio istituzionale della ricerca (Alma Mater Studiorum Università di Bologna) (submittedVersion)
  • University of Groningen research database (University of Groningen / Centre for Information Technology) (submittedVersion, other-oa)
  • arXiv (Cornell University)
Funders
  • European Space Agency
  • Nederlandse Organisatie voor Wetenschappelijk Onderzoek
Topics
  • Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Stellar, planetary, and galactic studies
  • Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Galaxies: Formation, Evolution, Phenomena
  • Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Astrophysics and Star Formation Studies
Keywords
globular clusters, Milky Way formation, galaxy mergers, Gaia-Enceladus, Sagittarius dwarf galaxy, Helmi streams, cluster kinematics, first integrals, cluster age, age-metallicity relation, binding energy

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