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

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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.

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.


  1. 1.

    Searle, L. & Zinn, R. Compositions of halo clusters and the formation of the galactic halo. Astrophys. J. 225, 357–379 (1978).

  2. 2.

    Springel, V. et al. Simulations of the formation, evolution and clustering of galaxies and quasars. Nature 435, 629–636 (2005).

  3. 3.

    Bullock, J. S. & Johnston, K. V. Tracing galaxy formation with stellar halos. I. Methods. Astrophys. J. 635, 931–949 (2005).

  4. 4.

    Cooper, A. P. et al. Galactic stellar haloes in the CDM model. Mon. Not. R. Astron. Soc. 406, 744–766 (2010).

  5. 5.

    Johnston, K. V. et al. Tracing galaxy formation with stellar halos. II. Relating substructure in phase and abundance space to accretion histories. Astrophys. J. 689, 936–957 (2008).

  6. 6.

    Veljanoski, J. et al. Kinematics of outer halo globular clusters in M31. Astrophys. J. Lett. 768, L33 (2013).

  7. 7.

    Veljanoski, J. et al. The outer halo globular cluster system of M31—II. Kinematics. Mon. Not. R. Astron. Soc. 442, 2929–2950 (2014).

  8. 8.

    Perrett, K. M. et al. The kinematics and metallicity of the M31 globular cluster system. Astron. J. 123, 2490–2510 (2002).

  9. 9.

    Caldwell, N. & Romanowsky, A. J. Star clusters in M31. VII. Global kinematics and metallicity subpopulations of the globular clusters. Astrophys. J. 824, 42 (2016).

  10. 10.

    Ferguson, A. M. N., Irwin, M. J., Ibata, R. A., Lewis, G. F. & Tanvir, N. R. Evidence for stellar substructure in the halo and outer disk of M31. Astron. J. 124, 1452–1463 (2002).

  11. 11.

    Ibata, R. A. et al. The haunted halos of Andromeda and Triangulum: a panorama of galaxy formation in action. Astrophys. J. 671, 1591–1623 (2007).

  12. 12.

    Ibata, R. A. et al. The large-scale structure of the halo of the Andromeda galaxy. I. Global stellar density, morphology and metallicity properties. Astrophys. J. 780, 128 (2014).

  13. 13.

    Mackey, A. D. et al. The outer halo globular cluster system of M31—III. Relationship to the stellar halo. Mon. Not. R. Astron. Soc. 484, 1756–1789 (2019).

  14. 14.

    Ibata, R. A. et al. A vast, thin plane of corotating dwarf galaxies orbiting the Andromeda galaxy. Nature 493, 62–65 (2013).

  15. 15.

    McConnachie, A. W. et al. The remnants of galaxy formation from a panoramic survey of the region around M31. Nature 461, 66–69 (2009).

  16. 16.

    McConnachie, A. W. et al. The large-scale structure of the halo of the Andromeda galaxy. II. Hierarchical structure in the Pan-Andromeda Archaeological Survey. Astrophys. J. 868, 55 (2018).

  17. 17.

    Huxor, A. P. et al. Globular clusters in the outer halo of M31: the survey. Mon. Not. R. Astron. Soc. 385, 1989–1997 (2008).

  18. 18.

    Huxor, A. P. et al. The outer halo globular cluster system of M31—I. The final PAndAS catalogue. Mon. Not. R. Astron. Soc. 442, 2165–2187 (2014).

  19. 19.

    Mackey, A. D. et al. Evidence for an accretion origin for the outer halo globular cluster system of M31. Astrophys. J. Lett. 717, L11–L16 (2010).

  20. 20.

    Ibata, R. A. et al. On the accretion origin of a vast extended stellar disk around the Andromeda galaxy. Astrophys. J. 634, 287–313 (2005).

  21. 21.

    Dorman, C. E. et al. A clear age-velocity dispersion correlation in Andromeda’s stellar disk. Astrophys. J. 803, 24 (2015).

  22. 22.

    Ibata, R. A., Irwin, M., Lewis, G., Ferguson, A. M. N. & Tanvir, N. A giant stream of metal-rich stars in the halo of the galaxy M31. Nature 412, 49–52 (2001).

  23. 23.

    Fardal, M. A. et al. Inferring the Andromeda galaxy’s mass from its giant southern stream with Bayesian simulation sampling. Mon. Not. R. Astron. Soc. 434, 2779–2802 (2013).

  24. 24.

    Hammer, F. et al. A 2–3 billion year old major merger paradigm for the Andromeda galaxy and its outskirts. Mon. Not. R. Astron. Soc. 475, 2754–2767 (2018).

  25. 25.

    D’Souza, R. & Bell, E. F. The Andromeda galaxy’s most important merger about 2 billion years ago as M32’s likely progenitor. Nature Astron. 2, 737–743 (2018).

  26. 26.

    Conn, A. R. et al. The three-dimensional structure of the M31 satellite system; strong evidence for an inhomogeneous distribution of satellites. Astrophys. J. 766, 120 (2013).

  27. 27.

    Fernando, N. et al. On the stability of satellite planes—I. Effects of mass, velocity, halo shape and alignment. Mon. Not. R. Astron. Soc. 465, 641–652 (2017).

  28. 28.

    Fernando, N., Arias, V., Lewis, G. F., Ibata, R. A. & Power, C. Stability of satellite planes in M31 II: effects of the dark subhalo population. Mon. Not. R. Astron. Soc. 473, 2212–2221 (2018).

  29. 29.

    Libeskind, N. I. et al. Planes of satellite galaxies and the cosmic web. Mon. Not. R. Astron. Soc. 452, 1052–1059 (2015).

  30. 30.

    Harris, W. E., Blakeslee, J. P. & Harris, G. L. H. Galactic dark matter halos and globular cluster populations. III. Extension to extreme environments. Astrophys. J. 836, 67 (2017).

  31. 31.

    Martin, N. F. et al. The PAndAS view of the Andromeda satellite system. I. A Bayesian search for dwarf galaxies using spatial and color-magnitude information. Astrophys. J. 776, 80 (2013).

  32. 32.

    Galleti, S., Federici, L., Bellazzini, M., Fusi Pecci, F. & Macrina, S. 2MASS NIR photometry for 693 candidate globular clusters in M 31 and the Revised Bologna Catalogue. Astron. Astrophys. 416, 917–924 (2004).

  33. 33.

    Caldwell, N. et al. Star clusters in M31. I. A catalog and a study of the young clusters. Astron. J. 137, 94–110 (2009).

  34. 34.

    Peacock, M. B. et al. The M31 globular cluster system: ugriz and K-band photometry and structural parameters. Mon. Not. R. Astron. Soc. 402, 803–818 (2010).

  35. 35.

    Caldwell, N., Schiavon, R., Morrison, H., Rose, J. A. & Harding, P. Star clusters in M31. II. Old cluster metallicities and ages from Hectospec data. Astron. J. 141, 61 (2011).

  36. 36.

    Dorman, C. E. et al. A new approach to detailed structural decomposition from the SPLASH and PHAT surveys: kicked-up disk stars in the Andromeda galaxy? Astrophys. J. 779, 103 (2013).

  37. 37.

    Brewer, B. J., Pártay, L. B. & Csányi, G. Diffusive nested sampling. Stat. Comput. 21, 649–656 (2011).

  38. 38.

    Brewer, B. J. & Foreman-Mackey, D. DNest4: diffusive nested sampling in C++ and Python. J. Stat. Softw. 86, 1–33 (2018).

  39. 39.

    Skilling, J. Nested sampling for general Bayesian computation. Bayesian Anal. 1, 833–859 (2006).

  40. 40.

    Foreman-Mackey, D. scatterplot matrices in Python. J. Open Source Softw. 1, 24 (2016).

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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.

Author information

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.

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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