2010MNRAS.404.1157M

We report the discovery of an extended globular cluster in a halo field in Centaurus A (NGC 5128), situated from the centre of that galaxy, imaged with the Advanced Camera for Surveys on board the Hubble Space Telescope. At the distance of the galaxy, the half-light radius of the cluster is r_h∼ 17pc, placing it among the largest globular clusters known. The faint absolute magnitude of the star cluster, M_V,{cir}_= -5.2, and its large size render this object somewhat different from the population of extended globular clusters previously reported, making it the first firm detection in the outskirts of a giant galaxy of an analogue of the faint, diffuse globular clusters present in the outer halo of the Milky Way. The colour-magnitude diagram of the cluster, covering approximately the brightest four magnitudes of the red giant branch, is consistent with an ancient, i.e. ≳8Gyr, intermediate metallicity, i.e. [M/H] ∼ -1.0dex, stellar population. We also report the detection of a second, even fainter cluster candidate which would have r_h∼ 9pc and M_V,{cir}_= -3.4 if it is at the distance of NGC 5128. The properties of the extended globular cluster and the diffuse stellar populations in its close vicinity suggest that they are part of a low mass accretion in the outer regions of NGC 5128.