The high mass end of extragalactic globular clusters

TL;DR

This paper reviews the properties of ultra-compact dwarf galaxies (UCDs) and globular clusters, discussing their similarities, differences, and possible origins, including whether UCDs are massive star clusters or dark matter dominated dwarf galaxies.

Contribution

It provides a comparative analysis of UCDs and globular clusters, highlighting key differences and discussing their potential origins in the context of galaxy formation.

Findings

UCDs are more massive and larger than typical globular clusters.

UCDs have higher mass-to-light ratios than GCs.

A possible break in scaling laws occurs around 2.5x10^6 M_sun.

Abstract

In the last decade, a new kind of stellar systems has been established that shows properties in between those of globular clusters (GCs) and early-type dwarf galaxies. These so-called ultra-compact dwarf galaxies (UCDs) have masses in the range 10^6 to 10^8 M_sun and half-light radii of 10-100 pc. The most massive UCDs known to date are predominantly metal-rich and reside in the cores of nearby galaxy clusters. The question arises whether UCDs are just the most massive globular clusters in rich globular cluster systems? Although UCDs and `normal' GCs form a continuous sequence in several parameter spaces, there seems to be a break in the scaling laws for stellar systems with masses above ~2.5x10^6 M_sun. Unlike GCs, UCDs follow a mass-size relation and their mass-to-light ratios are about twice as large as those of GCs with comparable metallicities. In this contribution, I present the properties of the brightest globular clusters and ultra-compact dwarf galaxies and discuss whether the observed findings are compatible with a `star-cluster' origin of UCDs or whether they are more likely related to dark matter dominated dwarf galaxies.