TUESDAY 19 August 2008
Tobias Goerdt (Hebrew University)
"The formation of ultra-compact dwarf galaxies and nucleated dwarf galaxies"
Ultra compact dwarf galaxies (UCDs) have similar properties as massive globular clusters or the
nuclei of nucleated galaxies. Recent observations suggesting a high dark matter content and a steep
spatial distribution within groups and clusters provide new clues as to their origins. We perform
high-resolution N-body / smoothed particle hydrodynamics simulations designed to elucidate two possible
formation mechanisms for these systems: the merging of globular clusters in the centre of a dark matter
halo, or the massively stripped remnant of a nucleated galaxy. Both models produce density profiles as
well as the half light radii that can fit the observational constraints. However, we show that the first
scenario results to UCDs that are underluminous and contain no dark matter. This is because the sinking
process ejects most of the dark matter particles from the halo centre. Stripped nuclei give a more
promising explanation, especially if the nuclei form via the sinking of gas, funneled down inner galactic
bars, since this process enhances the central dark matter content. Even when the entire disk is tidally
stripped away, the nucleus stays intact and can remain dark matter dominated even after severe stripping.
Total galaxy disruption beyond the nuclei only occurs on certain orbits and depends on the amount of
dissipation during nuclei formation. By comparing the total disruption of CDM subhaloes in a cluster
potential we demonstrate that this model also leads to the observed spatial distribution of UCDs which can
be tested in more detail with larger data sets.