Astronomy & Astrophysics is publishing a new statistical study of the so-called ‘ultra-compact dwarf galaxies’ (UCDs), which are still mysterious objects. A team of astronomers has investigated how many of these UCDs exist in nearby galaxy clusters and groups. They show that the properties of UCDs match those of 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.

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