In hierarchical cosmological models1, galaxies grow in mass through the continual accretion of smaller ones. The tidal disruption of these systems is expected to result in loosely bound stars surrounding the galaxy, at distances that reach 10–100 times the radius of the central disk2,3. The number, luminosity and morphology of the relics of this process provide significant clues to galaxy formation history4, but obtaining a comprehensive survey of these components is difficult because of their intrinsic faintness and vast extent. Here we report a panoramic survey of the Andromeda galaxy (M31). We detect stars and coherent structures that are almost certainly remnants of dwarf galaxies destroyed by the tidal field of M31. An improved census of their surviving counterparts implies that three-quarters of M31’s satellites brighter than Mv = -6 await discovery. The brightest companion, Triangulum (M33), is surrounded by a stellar structure that provides persuasive evidence for a recent encounter with M31. This panorama of galaxy structure directly confirms the basic tenets of the hierarchical galaxy formation model and reveals the shared history of M31 and M33 in the unceasing build-up of galaxies.

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We thank the entire staff at the Canada–France–Hawaii Telescope for taking the data, for initial processing with Elixir and for their continuing support throughout this project. A.M.N.F. and A.D.M. are supported by a Marie Curie Excellence Grant from the European Commission under contract MCEXT-CT-2005-025869. G.F.L. thanks the Australian Nuclear Science and Technology Organisation (ANSTO) for supporting his involvement in PAndAS through its Access to Major Research Facilities Program (AMRFP). R.M.R. acknowledges grant AST-0709479 from the National Science Foundation, and grants GO-9453, GO-10265 and GO-10816 from the Space Telescope Science Institute. The image of M33 overlaid in Fig. 1 is reproduced by courtesy of T. A. Rector and M. Hanna.

Author Contributions All authors assisted in the development and writing of the paper. In addition, A.W.M. is the Principal Investigator of PAndAS; M.J.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., A.W.M. and N.T.); J.D., L.M.W. modelled the M31–M33 interaction; N.F.M. had a lead role in the study of the dwarf galaxies; P.C. assisted with constructing the luminosity function; and A.L.D. developed the theoretical isochrones.

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  1. NRC Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, British Columbia, Canada V9E 2E7

    • Alan W. McConnachie
    • , Patrick Côté
    •  & Thomas H. Puzia
  2. Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK

    • Michael J. Irwin
    • , Scott C. Chapman
    • , Michelle L. M. Collins
    •  & Jorge Peñarrubia
  3. Observatoire de Strasbourg, 11, rue de l’Université, F-67000 Strasbourg, France

    • Rodrigo A. Ibata
    • , Olivier Bienaymé
    •  & Arnaud Siebert
  4. Department of Astronomy & Astrophysics, University of Toronto, 50 St George Street, Toronto, Ontario, Canada M5S 3H4

    • John Dubinski
  5. Department of Physics, Engineering Physics, and Astronomy Queen’s University, Kingston, Ontario, Canada K7L 3N6

    • Lawrence M. Widrow
  6. Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany

    • Nicolas F. Martin
  7. Department of Physics and Astronomy, University of Victoria, 3800 Finnerty Road, Victoria, British Columbia, Canada V8P 5C2

    • Aaron L. Dotter
    • , Julio F. Navarro
    • , Arif Babul
    •  & Kimberly A. Venn
  8. Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK

    • Annette M. N. Ferguson
    •  & A. Dougal Mackey
  9. Sydney Institute for Astronomy, School of Physics, University of Sydney, NSW 2006, Australia

    • Geraint F. Lewis
  10. Department of Physics and Astronomy, University of Western Ontario, 1151 Richmond Street, London, Ontario, Canada N6A 3K7

    • Pauline Barmby
  11. Department of Physics and Astronomy, McMaster University, Hamilton, Ontario, Canada L8S 4M1

    • Robert Cockcroft
    •  & William E. Harris
  12. University of Massachusetts, Department of Astronomy, LGRT 619-E, 710 N. Pleasant Street, Amherst, Massachusetts 01003-9305, USA

    • Mark A. Fardal
  13. Department of Physics (Astrophysics Group), H. H. Wills Physics Laboratory, Tyndall Avenue, Bristol BS8 1TL, UK

    • Avon Huxor
  14. Department of Physics and Astronomy, University of California, Los Angeles, PAB, 430 Portola Plaza, Los Angeles, California 90095-1547, USA

    • R. Michael Rich
  15. Department of Physics and Astronomy, 6224 Agricultural Road, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1

    • Harvey B. Richer
  16. Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK

    • Nial Tanvir
  17. Laboratoire Galaxies et Étoiles, Physique et Instrumentation, CNRS UMR 8111, Observatoire de Paris, 5 Place Jules Janssen, 92195 Meudon, France

    • David Valls-Gabaud


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Corresponding author

Correspondence to Alan W. McConnachie.

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PDF files

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    Supplementary Information

    This file contains a Supplementary Discussion, Supplementary Methods and Supplementary Tables 1-2.


  1. 1.

    Supplementary Movie 1

    This animated movie depicts a model of the interaction of M31 and M33 highlighting the excitation of tidal tails as the plausible origin of the extensions of the M33 stellar disk and gaseous warp. The tidal stripping of the M33 stellar halo is not the likely origin of the observed extensions.

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