The Formation of Ultra-Compact Dwarf Galaxies

Recent spectroscopic observations of galaxies in the Fornax-Cluster reveal nearly unresolved `star-like' objects with red-shifts appropriate to the Fornax-Cluster. These objects have intrinsic sizes of about 100 pc and absolute B-band magnitudes in the range -14 < M_B < -11.5 mag and lower limits for the central surface brightness mu_B > 23 mag/arcsec^2 (Phillipps et al. 2001), and so appear to constitute a new population of ultra-compact dwarf galaxies (UCDs). Such compact dwarfs were predicted to form from the amalgamation of stellar super-clusters (Kroupa 1998), which are rich aggregates of young massive star clusters (YMCs) that can form in collisions between gas-rich galaxies. Here we present the evolution of super-clusters in a tidal field. The YMCs merge on a few super-cluster crossing times. Super-clusters that are initially as concentrated and massive as knot~S in the interacting Antennae galaxies (Whitmore et al. 1999) evolve to merger objects that are long-lived and show properties comparable to the newly discovered UCDs. Less massive super-clusters resembling knot 430 in the Antennae may evolve to omega-Cen-type systems. Low-concentration super-clusters are disrupted by the tidal field, dispersing their surviving star clusters while the remaining merger objects rapidly evolve into the mu_B-M_B region populated by low-mass Milky-Way dSph satellites.