Origin of lower velocity dispersions of ultra-compact dwarf galaxy populations in clusters of galaxies

Recent observations have revealed that velocity dispersions of “ultra-compact dwarf” (UCD) galaxies are significantly smaller than those of other galaxy populations in the Fornax and the Virgo clusters of galaxies. In order to understand the origin of the observed lower velocity dispersions of UCDs, we numerically investigate line-of-sight velocity dispersion (σlos) of galaxy populations with variously different orbits in clusters of galaxies with the total masses of Mcl. We particularly investigate radial velocity dispersion profiles (σlos(R)) and velocity dispersions within the central 200 kpc of a cluster model (σm) for galaxies with different pericenter distances (rp) and orbital eccentricities (e) in the model with Mcl = 7.0×10 M⊙ reasonable for the Fornax cluster. We find that σlos(R) and σm of galaxies with smaller rp are steeper and smaller, respectively, for a given initial e distribution of galaxies. For example, we find that σm is ∼ 260 km s −1 for galaxies with rp < 50 kpc and ∼ 336 km s −1 for all galaxies in the model with the mean e of 0.6. These results imply that the observed lower velocity dispersion of UCD population is consistent with the UCDs having significantly smaller rp than other galaxy populations in the Fornax. We discuss these results in the context of the “galaxy threshing” scenario in which UCDs originate from nuclei of nucleated dwarf galaxies. We suggest that the observed differences in kinematical properties between UCDs and other dwarf galaxy populations in clusters of galaxies can be understood in terms of the differences in orbital properties between UCDs and the dwarf populations.