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Ultra-Compact Dwarf Galaxies Are Bright Star Clusters |

Astronomy & Astrophysics is publishing the results of a detailed investigation of how many ‘ultra-compact dwarf galaxies’ (UCDs) can be found in nearby galaxy clusters. UCDs were recognized as a populous and potentially distinct class of stellar systems about a decade ago. But they are still mysterious objects that 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) present a statistical study of the UCD population: they define new statistical tools that relate the number of UCDs to the total luminosity of their host environment. This allows them to use statistical arguments to test the hypothesis that UCDs are bright star clusters. They predict 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. continue reading

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