Abstract
We discuss the separation between UCDs and globular clusters (GCs) in the Fornax cluster. Ultra-compact dwarf galaxies (UCDs) have been proposed as a new class of galaxies, populating the central regions of the Fornax, Virgo and Abell 1689 clusters (Hilker et al. 1999, A&AS, 134, 75; Drinkwater et al. 2000, PASA, 17, 227; Hasegan et al. 2005, ApJ, 627, 203; Jones et al. 2006, AJ, 131, 312; Mieske et al. 2005, A&A, 430L, 25). UCDs are placed between the sequence of globular clusters and dwarf elliptical galaxies in the fundamental plane (Drinkwater et al. 2003, Nature, 423, 519), having absolute magnitudes M_V > -13.5 mag. Two UCD formation channels are discussed: 1) ``Super star clusters'' formed in galaxy mergers (Fellhauer & Kroupa 2002, MNRAS, 330, 642). 2) Remnant nuclei of dE,Ns stripped in the potential well of their host cluster (Bekki et al. 2003, MNRAS, 344, 399). In order to discuss those scenarios properly, one first needs to cleanly separate UCDs from the morphologically similar globular clusters (GCs). Regarding this issue, in Mieske et al. (2006, AJ in press, astro-ph/0512474) we find two distinctive breaks in the properties of Fornax compact objects, i.e. within a joint sample of UCDs plus GCs. Both breaks occur at M_V ≃ -11 mag (3*10^6 Msun): 1. In the metallicity-luminosity plane, objects with M_V < -11 mag have a narrow metallicity distribution centred around [Fe/H]=-0.6 dex. Objects with M_V > -11 mag have a broader metallicity distribution centred around -1.2 dex. 2. In the size-luminosity plane, objects with M_V < -11 mag have half-light radii that positively correlate with luminosity. Objects with M_V > -11 mag have luminosity independent half-light radii around 2-5 pc, typical for GCs. We therefore conclude that in Fornax the separation between GCs and UCDs occurs at ≈ 3*10^6 Msun (M_V = -11 mag).