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Two major accretion epochs in M31 from two distinct populations of globular clusters


Large galaxies grow through the accumulation of dwarf galaxies1,2. In principle it is possible to trace this growth history via the properties of a galaxy’s stellar halo3,4,5. Previous investigations of the galaxy Messier 31 (M31, Andromeda) have shown that outside a galactocentric radius of 25 kiloparsecs the population of halo globular clusters is rotating in alignment with the stellar disk6,7, as are more centrally located clusters8,9. The M31 halo also contains coherent stellar substructures, along with a smoothly distributed stellar component10,11,12. Many of the globular clusters outside a radius of 25 kiloparsecs are associated with the most prominent substructures, but some are part of the smooth halo13. Here we report an analysis of the kinematics of these globular clusters. We find two distinct populations rotating perpendicular to each other. The rotation axis for the population associated with the smooth halo is aligned with the rotation axis for the plane of dwarf galaxies14 that encircles M31. We interpret these separate cluster populations as arising from two major accretion epochs, probably separated by billions of years. Stellar substructures from the first epoch are gone, but those from the more recent second epoch still remain.

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Fig. 1: Map showing the distribution of metal-poor red giant stars in Andromeda’s halo.
Fig. 2: Best-fit rotational kinematics for the favoured model V2.

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Data availability

All data analysed for this study are publicly available. Pan-Andromeda Archaeological Survey data products, including the stellar photometry catalogue, reduced individual images, and image stacks, may be downloaded from the Canadian Astronomical Data Center (CADC; Globular cluster locations, radial velocities, and classifications are published online7,13,18 and are also found in the code repository (

Code availability

Readers may access the code used for the inference, with the data included, at The code has been released under the permissive MIT licence. The README file describes how to reproduce the results of the paper.


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This work is based in part on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT), which is operated by the National Research Council (NRC) of Canada, the Institut National des Sciences de l’Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii. This work is further based in part on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the National Research Council (Canada), CONICYT (Chile), the Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina), the Ministério da Ciência, Tecnologia e Inovação (Brazil) and the Korea Astronomy and Space Science Institute (Republic of Korea). Some of the data presented here were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. We wish to recognise and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. The William Herschel Telescope (WHT) is operated on the island of La Palma by the Isaac Newton Group of Telescopes in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. This work is based in part on observations at Kitt Peak National Observatory, National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA) under cooperative agreement with the National Science Foundation. We are honoured to be permitted to conduct astronomical research on Iolkam Du’ag (Kitt Peak), a mountain with particular significance to the Tohono O’odham. D.M. is supported by an Australian Research Council (ARC) Future Fellowship (FT160100206). G.F.L. acknowledges support from a Partnership Collaboration Award between the University of Sydney and the University of Edinburgh. D.M. and G.F.L. appreciate the hospitality of the Royal Observatory, Edinburgh, where the final stages of the preparation of this paper were undertaken. B.J.B. thanks the Marsden Fund of the Royal Society of New Zealand. This work has been published under the framework of the IdEx Unistra and benefits from funding from the state managed by the French National Research Agency as part of the investments for the future program. Z.W. is supported by a Dean’s International Postgraduate Research Scholarship at the University of Sydney.

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Authors and Affiliations



A.W.M., R.A.I., M.J.I., A.M.N.F., G.F.L. and N.T. initiated the Pan-Andromeda Archaeological Survey, with extensive data analysis and interpretation undertaken with N.M., M.L.M.C., P.C. and J.P. D.M., A.M.N.F., J.V. and A.P.H. were responsible for the discovery and characterization of Pan-Andromeda Archaeological Survey globular clusters, for measuring their line-of-sight velocities, and for conducting detailed earlier analyses of this population. G.F.L. was responsible for the development of the kinematic models employed in this study. G.F.L. and B.J.B. undertook the statistical analysis, with B.J.B. responsible for implementing and running the kinematic models in DNest4. Z.W. was responsible for undertaking geometric transformations into the Andromeda frame to enable comparisons with previous studies. All authors assisted in the interpretation of the results and writing of the paper.

Corresponding author

Correspondence to Dougal Mackey.

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Extended data figures and tables

Extended Data Fig. 1 Corner plot of the posterior distribution of the parameters for the V2 model.

This model is defined in Extended Data Table 1. Subscripts 0 and 1 correspond to the parameters for the non-substructure (GC-Non) and substructure (GC-Sub) samples respectively, and a summary of the marginal distributions is given in Extended Data Table 4. Corner plots were generated using Python40.

Extended Data Table 1 Functional form of the rotational component for the models under consideration
Extended Data Table 2 The joint prior distribution for all parameters and data
Extended Data Table 3 Marginal likelihoods for each of the models, along with the resulting Bayes factor compared to the most favoured model V2
Extended Data Table 4 Estimates of the parameters for the favoured model V2

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Mackey, D., Lewis, G.F., Brewer, B.J. et al. Two major accretion epochs in M31 from two distinct populations of globular clusters. Nature 574, 69–71 (2019).

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