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

*Corresponding author for this work

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

81 Citations (Scopus)


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 ∼60 kpc from M31. Our F606W, F814W color-magnitude diagrams extend ∼3 mag below the horizontal branch level, and clearly demonstrate, for the first time, that all four clusters are composed of ≳ 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 ≲ [Fe/H] ≲ -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 ≲ Mv ≲ -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
JournalAstrophysical Journal
Issue number2 II
Publication statusPublished - 20 Dec 2006


  • Galaxies: individual (M31)
  • Galaxies: star clusters
  • Globular clusters: general


Dive into the research topics of 'ACS photometry of extended, luminous globular clusters in the outskirts of M31'. Together they form a unique fingerprint.

Cite this