| Autor: |
Mackey D; Research School of Astronomy and Astrophysics, Australian National University, Canberra, Australian Capital Territory, Australia. dougal.mackey@anu.edu.au., Lewis GF; Sydney Institute for Astronomy, School of Physics, The University of Sydney, Sydney, New South Wales, Australia., Brewer BJ; Department of Statistics, The University of Auckland, Auckland, New Zealand., Ferguson AMN; Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh, UK., Veljanoski J; Kapteyn Astronomical Institute, University of Groningen, Groningen, The Netherlands., Huxor AP; HH Wills Physics Laboratory, University of Bristol, Bristol, UK., Collins MLM; Department of Physics, University of Surrey, Guildford, UK., Côté P; NRC Herzberg Astronomy and Astrophysics, Victoria, British Columbia, Canada., Ibata RA; Observatoire Astronomique de Strasbourg, Université de Strasbourg, CNRS UMR 7550, Strasbourg, France., Irwin MJ; Institute of Astronomy, University of Cambridge, Cambridge, UK., Martin N; Observatoire Astronomique de Strasbourg, Université de Strasbourg, CNRS UMR 7550, Strasbourg, France.; Max-Planck-Institut für Astronomie, Heidelberg, Germany., McConnachie AW; NRC Herzberg Astronomy and Astrophysics, Victoria, British Columbia, Canada., Peñarrubia J; Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh, UK., Tanvir N; Department of Physics and Astronomy, University of Leicester, Leicester, UK., Wan Z; Sydney Institute for Astronomy, School of Physics, The University of Sydney, Sydney, New South Wales, Australia. |
| Abstrakt: |
Large galaxies grow through the accumulation of dwarf galaxies 1,2 . In principle it is possible to trace this growth history via the properties of a galaxy's stellar halo 3-5 . 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 disk 6,7 , as are more centrally located clusters 8,9 . The M31 halo also contains coherent stellar substructures, along with a smoothly distributed stellar component 10-12 . 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 halo 13 . 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 galaxies 14 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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