ACS photometry of extended, luminous globular clusters in the outskirts of M31

A. D. Mackey, A. Huxor, A. M. N. Ferguson, N. R. Tanvir, M. Irwin, R. Ibata, T. Bridges, R. A. Johnson, G. Lewis

Research output: Contribution to journalArticlepeer-review


A new population of extended, luminous globular clusters has recently been discovered in the outskirts of M31. These objects have luminosities typical of classical globular clusters, but much larger half-light radii. We report the first results from deep ACS imaging of four such clusters, one of which is a newly discovered example lying at a projected distance of similar to 60 kpc from M31. Our F606W, F814W color-magnitude diagrams extend similar to 3 mag below the horizontal branch level, and clearly demonstrate, for the first time, that all four clusters are composed of greater than or similar to 10 Gyr old, metal-poor stellar populations. No evidence for multiple populations is observed. From a comparison with Galactic globular cluster fiducials we estimate metallicities in the range -2.2 less than or similar to [Fe/H] less than or similar to -1.8. The observed horizontal branch morphologies show a clear second parameter effect between the clusters. Preliminary radial luminosity profiles suggest integrated magnitudes in the range -7.7 less than or similar to M-v less than or similar to -6.6, near the median value of the globular cluster luminosity function. Our results confirm that these four objects are bona fide old, metal-poor globular clusters, albeit with combined structures and luminosities unlike those observed for any other globular clusters in the Local Group or beyond.

Original languageEnglish
Pages (from-to)L105-L108
Number of pages4
JournalThe Astrophysical Journal
Issue number2
Publication statusPublished - 20 Dec 2006


  • galaxies : individual (M31)
  • galaxies : star clusters
  • globular clusters : general


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