ultracompact

Ultracompact is a term used in astronomy and related fields to describe objects with exceptionally small radii and high densities for their mass. In astrophysics it most often refers to ultracompact dwarf galaxies, a class of compact stellar systems that occupy a space between the largest globular clusters and the smallest dwarf galaxies in terms of size and luminosity.

Ultracompact dwarf galaxies are characterized by radii on the order of tens of parsecs and masses that can approach those of dwarf galaxies, yet with stellar densities comparable to or exceeding those of globular clusters. They are typically brighter and more massive than ordinary globular clusters, and their kinematics sometimes yield high dynamical mass-to-light ratios. Their stellar populations can be diverse, exhibiting a range of metallicities and ages.

Several formation pathways are discussed in the literature. One leading scenario is tidal stripping in dense environments, where interactions with larger galaxies remove the outer parts of a nucleated dwarf galaxy, leaving a compact, dense remnant that is observed as a UCD. An alternative is that some UCDs form as unusually massive star clusters, perhaps through the merging of multiple clusters in starburst episodes. It is also possible that a subset consists of the stripped nuclei of galaxies that contained central massive black holes.

UCDs have been found in a variety of galaxy clusters and groups, notably in the Fornax and Virgo clusters, and are studied to illuminate galaxy assembly, tidal interactions, and the dynamics of dense stellar systems. See also globular cluster, dwarf galaxy, tidal stripping, and dense stellar systems.