1. NRC Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, British Columbia, Canada V9E 2E7
2. Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
3. Observatoire de Strasbourg, 11, rue de l'Université, F-67000 Strasbourg, France
4. Department of Astronomy & Astrophysics, University of Toronto, 50 St George Street, Toronto, Ontario, Canada M5S 3H4
5. Department of Physics, Engineering Physics, and Astronomy Queen's University, Kingston, Ontario, Canada K7L 3N6
6. Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
7. Department of Physics and Astronomy, University of Victoria, 3800 Finnerty Road, Victoria, British Columbia, Canada V8P 5C2
8. Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
9. Sydney Institute for Astronomy, School of Physics, University of Sydney, NSW 2006, Australia
10. Department of Physics and Astronomy, University of Western Ontario, 1151 Richmond Street, London, Ontario, Canada N6A 3K7
11. Department of Physics and Astronomy, McMaster University, Hamilton, Ontario, Canada L8S 4M1
12. University of Massachusetts, Department of Astronomy, LGRT 619-E, 710 N. Pleasant Street, Amherst, Massachusetts 01003-9305, USA
13. Department of Physics (Astrophysics Group), H. H. Wills Physics Laboratory, Tyndall Avenue, Bristol BS8 1TL, UK
14. Department of Physics and Astronomy, University of California, Los Angeles, PAB, 430 Portola Plaza, Los Angeles, California 90095-1547, USA
15. Department of Physics and Astronomy, 6224 Agricultural Road, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1
16. Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
17. Laboratoire Galaxies et Étoiles, Physique et Instrumentation, CNRS UMR 8111, Observatoire de Paris, 5 Place Jules Janssen, 92195 Meudon, France

Correspondence to: Alan W. McConnachie¹ Correspondence and requests for materials should be addressed to A.W.M. (Email: alan.mcconnachie@nrc-cnrc.gc.ca).

Abstract

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 disk^{2, 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 M_v = -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.

1. NRC Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, British Columbia, Canada V9E 2E7
2. Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
3. Observatoire de Strasbourg, 11, rue de l'Université, F-67000 Strasbourg, France
4. Department of Astronomy & Astrophysics, University of Toronto, 50 St George Street, Toronto, Ontario, Canada M5S 3H4
5. Department of Physics, Engineering Physics, and Astronomy Queen's University, Kingston, Ontario, Canada K7L 3N6
6. Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
7. Department of Physics and Astronomy, University of Victoria, 3800 Finnerty Road, Victoria, British Columbia, Canada V8P 5C2
8. Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
9. Sydney Institute for Astronomy, School of Physics, University of Sydney, NSW 2006, Australia
10. Department of Physics and Astronomy, University of Western Ontario, 1151 Richmond Street, London, Ontario, Canada N6A 3K7
11. Department of Physics and Astronomy, McMaster University, Hamilton, Ontario, Canada L8S 4M1
12. University of Massachusetts, Department of Astronomy, LGRT 619-E, 710 N. Pleasant Street, Amherst, Massachusetts 01003-9305, USA
13. Department of Physics (Astrophysics Group), H. H. Wills Physics Laboratory, Tyndall Avenue, Bristol BS8 1TL, UK
14. Department of Physics and Astronomy, University of California, Los Angeles, PAB, 430 Portola Plaza, Los Angeles, California 90095-1547, USA
15. Department of Physics and Astronomy, 6224 Agricultural Road, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1
16. Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
17. Laboratoire Galaxies et Étoiles, Physique et Instrumentation, CNRS UMR 8111, Observatoire de Paris, 5 Place Jules Janssen, 92195 Meudon, France

Correspondence to: Alan W. McConnachie¹ Correspondence and requests for materials should be addressed to A.W.M. (Email: alan.mcconnachie@nrc-cnrc.gc.ca).

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