Young accreted globular clusters in the outer halo of m31

A. D. Mackey*, A. P. Huxor, A. M.N. Ferguson, M. J. Irwin, J. Veljanoski, A. W. McConnachie, R. A. Ibata, G. F. Lewis, N. R. Tanvir

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

38 Citations (Scopus)


We report on observations of two newly discovered globular clusters in the outskirts of M31 made using the Gemini Multi-Object Spectrograph (GMOS) instrument on Gemini North. These objects, PAndAS-7 (PA-7) and PAndAS-8 (PA-8), lie at a galactocentric radius of 87 kpc and are projected, with separation 19 kpc, on to a field halo substructure known as the South-West Cloud. We measure radial velocities for the two clusters which confirm that they are almost certainly physically associated with this feature. Colour-magnitude diagrams reveal strikingly short, exclusively red horizontal branches in both PA-7 and PA-8; both also have photometric [Fe/H]=-1.35±0.15. At this metallicity, the morphology of the horizontal branch is maximally sensitive to age, and we use the distinctive configurations seen in PA-7 and PA-8 to demonstrate that both objects are very likely to be at least 2 Gyr younger than the oldest Milky Way globular clusters. Our observations provide strong evidence for young globular clusters being accreted into the remote outer regions of M31 in a manner entirely consistent with the established picture for the Milky Way, and add credence to the idea that similar processes play a central role in determining the composition of globular cluster systems in large spiral galaxies in general.

Original languageEnglish
Pages (from-to)281-293
Number of pages13
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
Publication statusPublished - 22 Feb 2013


  • General - galaxies
  • Globular clusters
  • Haloes - Local Group
  • Individual
  • M31 - galaxies


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