Journal
ASTROPHYSICAL JOURNAL
Volume 653, Issue 2, Pages L105-L108Publisher
UNIV CHICAGO PRESS
DOI: 10.1086/510670
Keywords
galaxies : individual (M31); galaxies : star clusters; globular clusters : general
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Funding
- STFC [PP/D000920/1, PP/C50352X/1] Funding Source: UKRI
- Science and Technology Facilities Council [PP/C50352X/1, PP/D000920/1] Funding Source: researchfish
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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.
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