UCDs - a mixed bag of objects

TL;DR

This paper reviews the discovery, properties, and possible origins of Ultra-Compact Dwarf Galaxies (UCDs), discussing their formation scenarios and implications based on current astronomical observations.

Contribution

It provides a comprehensive review of UCDs, comparing formation hypotheses and summarizing their observational characteristics in the context of galaxy evolution.

Findings

UCDs are more massive and extended than globular clusters.

Two main formation scenarios: remnant nuclei of disrupted galaxies and formation from merged star clusters.

UCDs' properties fit within the fundamental plane of stellar systems.

Abstract

The name "Ultra-Compact Dwarf Galaxy" (UCD) was invented for a new type of astronomical object that has been discovered in cores of nearby galaxy clusters a decade ago. UCDs resemble globular clusters, but are up to 100 times more massive and slightly more extended. Their luminosities are comparable to those of nuclei of the most massive dwarf ellipticals or late-type spirals. Various formation scenarios have been brought forward to explain the origin and evolution of UCDs. Two of them seem to be most promising: first, UCDs might be the remnant nuclei of galaxies that have been disrupted in the cluster environment. Second, UCDs might have formed from the agglomeration of many young, massive star clusters that were created during ancient merger events. In this contribution, I review the discovery history of UCDs, summarize their observational properties in the fundamental plane of stellar systems and discuss the implications for the different formation scenarios based on our current knowledge about these enigmatic objects.