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

A new population of extended, luminous globular clusters has recently beendiscovered in the outskirts of M31. These objects have luminosities typical ofclassical globular clusters, but much larger half-light radii. We report thefirst results from deep ACS imaging of four such clusters, one of which is anewly-discovered example lying at a projected distance of ~60 kpc from M31. OurF606W, F814W colour-magnitude diagrams extend ~3 magnitudes below thehorizontal branch level, and clearly demonstrate, for the first time, that allfour clusters are composed of >10 Gyr old, metal-poor stellar populations. Noevidence for multiple populations is observed. From a comparison with Galacticglobular cluster fiducials we estimate metallicities in the range -2.2 < [Fe/H]< -1.8. The observed horizontal branch morphologies show a clear secondparameter effect between the clusters. Preliminary radial luminosity profilessuggest integrated magnitudes in the range -6.6 < M_V < -7.7, near the medianvalue of the globular cluster luminosity function. Our results confirm thatthese four objects are bona fide old, metal-poor globular clusters, albeit withcombined structures and luminosities unlike those observed for any otherglobular clusters in the Local Group or beyond.Comment: 12 pages, 3 figures, 1 table; accepted for publication in ApJ Letter