Dwarf satellite galaxies are thought to be the remnants of the population of primordial structures that coalesced to form giant galaxies like the Milky Way1. It has previously been suspected2 that dwarf galaxies may not be isotropically distributed around our Galaxy, because several are correlated with streams of H i emission, and may form coplanar groups3. These suspicions are supported by recent analyses4,5,6,7. It has been claimed7 that the apparently planar distribution of satellites is not predicted within standard cosmology8, and cannot simply represent a memory of past coherent accretion. However, other studies dispute this conclusion9,10,11. Here we report the existence of a planar subgroup of satellites in the Andromeda galaxy (M 31), comprising about half of the population. The structure is at least 400 kiloparsecs in diameter, but also extremely thin, with a perpendicular scatter of less than 14.1 kiloparsecs. Radial velocity measurements12,13,14,15 reveal that the satellites in this structure have the same sense of rotation about their host. This shows conclusively that substantial numbers of dwarf satellite galaxies share the same dynamical orbital properties and direction of angular momentum. Intriguingly, the plane we identify is approximately aligned with the pole of the Milky Way’s disk and with the vector between the Milky Way and Andromeda.

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We thank the staff of the Canada-France-Hawaii Telescope for taking the PAndAS data, and for their continued support throughout the project. We thank one of our referees, B. Tully, for pointing out that IC 1613 could also be associated to the planar structure. R.A.I. and D.V.G. gratefully acknowledge support from the Agence Nationale de la Recherche though the grant POMMME, and would like to thank B. Famaey for discussions. G.F.L. thanks the Australian Research Council for support through his Future Fellowship and Discovery Project. This work is based on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada–France–Hawaii Telescope, 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 (CNRS) of France, and the University of Hawaii. 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.

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  1. Observatoire Astronomique de Strasbourg, 11 rue de l’Université, F-67000 Strasbourg, France

    • Rodrigo A. Ibata
    •  & Nicolas F. Martin
  2. Sydney Institute for Astronomy, School of Physics, A28, The University of Sydney, New South Wales 2006, Australia

    • Geraint F. Lewis
  3. Department of Physics and Astronomy, Macquarie University, New South Wales 2109, Australia

    • Anthony R. Conn
  4. Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK

    • Michael J. Irwin
  5. NRC Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, British Columbia, V9E 2E7, Canada

    • Alan W. McConnachie
  6. Department of Physics and Atmospheric Science, Dalhousie University, 6310 Coburg Road, Halifax, Nova Scotia, B3H 4R2, Canada

    • Scott C. Chapman
  7. Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

    • Michelle L. Collins
    •  & Nicolas F. Martin
  8. University of Massachusetts, Department of Astronomy, LGRT 619-E, 710 North Pleasant Street, Amherst, Massachusetts 01003-9305, USA

    • Mark Fardal
  9. Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK

    • Annette M. N. Ferguson
  10. Lycée International des Pontonniers, 1 rue des Pontonniers, F-67000 Strasbourg, France

    • Neil G. Ibata
  11. The Australian National University, Mount Stromlo Observatory, Cotter Road, Weston Creek, Australian Capital Province 2611, Australia

    • A. Dougal Mackey
  12. Department of Physics and Astronomy, University of Victoria, 3800 Finnerty Road, Victoria, British Columbia, V8P 5C2, Canada

    • Julio Navarro
  13. Department of Physics and Astronomy, University of California, Los Angeles, PAB, 430 Portola Plaza, Los Angeles, California 90095-1547, USA

    • R. Michael Rich
  14. LERMA, UMR CNRS 8112, Observatoire de Paris, 61 Avenue de l’Observatoire, 75014 Paris, France

    • David Valls-Gabaud
  15. Department of Physics, Engineering Physics, and Astronomy, Queen’s University, Kingston, Ontario, K7L 3N, Canada

    • Lawrence M. Widrow


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All authors assisted in the development and writing of the paper. In addition, the structural and kinematic properties of the dwarf population, and the significance of the Andromeda plane were determined by R.A.I., G.F.L. and A.R.C., based on distances determined by the same group (as part of the PhD research of A.R.C.). In addition, A.W.M. is the Principal Investigator of PAndAS; M.J.I. and R.A.I. led the data processing effort; R.A.I. was the Principal Investigator of an earlier CFHT MegaPrime/MegaCam survey, which PAndAS builds on (which included S.C.C., A.M.N.F., M.J.I., G.F.L., N.F.M. and A.W.M.). R.M.R. is Principal Investigator of the spectroscopic follow-up with the Keck Telescope. M.L.C. and S.C.C. led the analysis of the kinematic determination of the dwarf population, and N.F.M. led the detection of the dwarf population from PAndAS data. N.G.I. performed the initial analysis of the satellite kinematics.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Rodrigo A. Ibata.

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