doi:10.1038/nindia.2013.86 Published online 27 June 2013
With the help of mathematical models and observations, researchers have obtained new insights into the origins and structures of ultracompact dwarf galaxies (UCDs) and ultrafaint dwarf galaxies (UFDs) . They found that these galaxies are bigger than, but have the same metal contents as, galactic globular clusters (GCs) — rich, compact, nearly spherical groups comprising hundreds of thousands (or even millions) of stars.
Among various hot stellar systems, the most enigmatic class of stellar systems is UCDs. They are a recently discovered class of very compact galaxies; they are of the order of 200 light years across and contain hundreds of millions of stars.
Various theories exist to explain their physical properties. Some consider them to be luminous extensions of GCs. Attempts have been made explore the origin of UCDs by seeking to unite old hot stellar systems from GCs and UCDs. However, the origins and structures of UCDs remain poorly understood.
To throw new light on them (including UFDs, which are cousins of UCDs that contain fewer stars), the researchers studied 694 hot stellar objects — GCs, UCDs, dwarf globular transition objects and other cosmic objects. They identified five different groups, which they named MK1 to MK5.
Both MK3 and MK4 contain UCDs and the nuclei of other cosmic objects. UCDs in MK3 might constitute the high-mass tail of GCs. Several formation channels of UCDs emerged. One is for UCDs in MK3. "UFDs could readily be classified as extended star clusters, rather than small dwarf galaxies. The internal velocity dispersions of the UFDs are quite elevated, implying they contain lots of dark matter," says Tanuka Chattopadhyay, a co-author of the study.