ACS photometry of extended, luminous globular clusters in the outskirts of M31

Journal article

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. © 2006. The American Astronomical Society. All rights reserved.

Authors: Mackey, A; Huxor, A; Ferguson, AMN; Tanvir, N; Irwin, M

• Publication status: Published
• Publisher: Institute of Physics Publishing
• Journal: ASTROPHYSICAL JOURNAL
• Volume: 653
• Issue: 2
• Pages: L105-L108
• Publication date: 2006-12-20
• DOI: 10.1086/510670
• EISSN: 1538-4357
• ISSN: 0004-637X
• Language: English
• Keywords: globular clusters : general; galaxies : individual (M31); galaxies : star clusters