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

Dougal Mackey (), Geraint F. Lewis, Brendon J. Brewer, Annette M. N. Ferguson, Jovan Veljanoski, Avon P. Huxor, Michelle L. M. Collins, Patrick Côté, Rodrigo A. Ibata, Mike J. Irwin, Nicolas Martin, Alan W. McConnachie, Jorge Peñarrubia, Nial Tanvir and Zhen Wan
Additional contact information
Dougal Mackey: Australian National University
Geraint F. Lewis: The University of Sydney
Brendon J. Brewer: The University of Auckland
Annette M. N. Ferguson: University of Edinburgh, Royal Observatory
Jovan Veljanoski: University of Groningen
Avon P. Huxor: University of Bristol
Michelle L. M. Collins: University of Surrey
Patrick Côté: NRC Herzberg Astronomy and Astrophysics
Rodrigo A. Ibata: Université de Strasbourg
Mike J. Irwin: University of Cambridge
Nicolas Martin: Université de Strasbourg
Alan W. McConnachie: NRC Herzberg Astronomy and Astrophysics
Jorge Peñarrubia: University of Edinburgh, Royal Observatory
Nial Tanvir: University of Leicester
Zhen Wan: The University of Sydney

Nature, 2019, vol. 574, issue 7776, 69-71

Abstract: Abstract 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 halo3–5. 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 component10–12. 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.

Date: 2019
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DOI: 10.1038/s41586-019-1597-1

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