Multi-colour Imaging of Ultra-compact Objects in the Fornax Cluster

Ultra-compact dwarf galaxies (UCDs) are a new type of galaxy we have discovered in the central region of the Fornax and Virgo clusters. Unresolved in around-based imaging, UCDs have spectra typical of old stellar systems. Ninety-two have been found in Fornax [1, 3], making them the most numerous galaxy type in the cluster. Although they form a cluster wide population, an over-density surrounds the central cluster galaxy, NGC1399, fueling controversy over their nature and origin. Several formation scenarios have been proposed. UCDs may be the remnant nuclei of tidally stripped dE,N galaxies [2] or they may be the bright tail of the globular cluster (CC) population associated with NGC1399 [3,"UCos"]. Alternatively they may be the first spectroscopically confirmed intracluster globular clusters (IGCs) in Fornax, resulting from hierarchical star cluster formation [4] and merging in intracluster space. The 5 brightest Fornax UCDs have M/L ratios indicating at least some dark matter, unlike typical GCs.