Astrophys. J., 653, L105-L108 (2006/December-3)
ACS photometry of extended, luminous globular clusters in the outskirts of M 31.
MacKEY A.D., HUXOR A., FERGUSON A.M.N., TANVIR N.R., IRWIN M., IBATA R., BRIDGES T., JOHNSON R.A. and LEWIS G.
Abstract (from CDS):
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 ≳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≲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.
Galaxies: Individual: Messier Number: M31 - Galaxies: Star Clusters - Galaxy: Globular Clusters: General
Table 1: M31WFS CN (No. C4) added.
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