The majority of ultra-compact dwarf galaxies, (UCDs), which are mysterious objects characterized by high stellar population counts, are actually very massive clusters of bright stars, a new study has revealed.
UCDs can be found in nearby galaxy clusters and are characterized by a compact morphology (30-300 light-years in size) and high masses (more than one million solar masses). More generally, their properties (e.g., their size, shape, or luminosity) are similar to those of both star clusters and dwarf galaxies.
Several hundred UCDs have been found to date. Two main formation channels for these puzzling objects have been proposed so far. UCDs might either be very massive star clusters or be 'normal' dwarf galaxies transformed by tidal effects.
S. Mieske, M. Hilker, and I. Misgeld (ESO) have defined new statistical tools that relate the number of UCDs to the total luminosity of their host environment.
This allowed them to use statistical arguments to test the hypothesis that UCDs are bright star clusters. They predicted that if UCDs are bright star clusters, we would expect to find only one or two UCDs around the Milky Way, which corresponds to what is seen, as omega Centauri is the only Milky Way satellite that can be considered a UCD.
With their new tools at hand, the authors investigated the results of spectroscopic UCD surveys in the environment of massive galaxy clusters such as Fornax, Hydra, and Centaurus, as well as in galaxy groups like our very own Local Group.
Most data analysed were obtained at ESO's Very Large Telescope in Chile, using the instruments FLAMES, VIMOS, and FORS2. For the Fornax cluster, they also used a large database of compact stellar objects, which contained about 180 UCDs.
This statistical study clearly showed that the luminosity distribution of UCDs matched very well with that of the brightest globular star clusters. It supported the hypothesis that the most of of UCDs are in fact very massive star clusters. From a statistical point of view, there is no need to invoke another scenario to explain the origin of UCDs.
The study has been published in Astronomy and Astrophysics.