A faint extended cluster in the outskirts of NGC 5128: evidence of a low mass accretion★

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 rh∼ 17 pc, placing it among the largest globular clusters known. The faint absolute magnitude of the star cluster, MV,○=−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. ≳8 Gyr, intermediate metallicity, i.e. [M/H]∼−1.0 dex, stellar population. We also report the detection of a second, even fainter cluster candidate which would have rh∼ 9 pc and MV,○=−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.