ACS Photometry of Extended, Luminous Globular Clusters in the Outskirts of M31

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 whi...

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Veröffentlicht in:The Astrophysical journal Jg. 653; H. 2; S. L105 - L108
Hauptverfasser: Mackey, A. D, Huxor, A, Ferguson, A. M. N, Tanvir, N. R, Irwin, M, Ibata, R, Bridges, T, Johnson, R. A, Lewis, G
Format: Journal Article
Sprache:Englisch
Veröffentlicht: Chicago, IL IOP Publishing 20.12.2006
University of Chicago Press
Schlagworte:
Astronomy
Earth, ocean, space
Exact sciences and technology
Galaxy clusters
galaxies: individual (M31)-galaxies: star clusters-globular clusters: general
Galaxies
Globular clusters
Star clusters
Photometry
ISSN:1538-4357, 0004-637X, 1538-4357
Online-Zugang:Volltext
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Zusammenfassung: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 660 kpc from M31. Our F606W, F814W color-magnitude diagrams extend 63 mag below the horizontal branch level, and clearly demonstrate, for the first time, that all four clusters are composed of 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 [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 M sub(v) -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.
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ISSN:1538-4357
0004-637X
1538-4357
DOI:10.1086/510670