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 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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