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

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

2006-11-08

arxiv.org/abs/astro-ph/0611257v1

Astrophys.J.653:L105-L108,2006

Astronomy and Astrophysics

Astrophysics

12 pages, 3 figures, 1 table; accepted for publication in ApJ Letters

Scientific paper

10.1086/510670

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 colour-magnitude diagrams extend ~3 magnitudes 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 -6.6 < M_V < -7.7, 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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