Mapping out the origins of compact stellar systems

TL;DR

This paper explores the origins of globular clusters and ultra-compact dwarfs, revealing their formation epochs, metallicity differences, and diverse evolutionary pathways, including evidence of stripped galaxy nuclei.

Contribution

It introduces metallicity-matching methods to trace the birth sites of GCs and uncovers a diverse range of compact stellar systems with varied origins.

Findings

Metallicity bimodality reflects distinct stellar populations.

Metal-rich GCs likely formed in clumpy disk galaxies at z ~ 3.

Many UCDs originate as stripped nuclei of larger galaxies.

Abstract

We present a suite of extragalactic explorations of the origins and nature of globular clusters (GCs) and ultra-compact dwarfs (UCDs), and the connections between them. An example of GC metallicity bimodality is shown to reflect underlying, distinct metal-poor and metal-rich stellar halo populations. Metallicity-matching methods are used to trace the birth sites and epochs of GCs in giant E/S0s, pointing to clumpy disk galaxies at z ~ 3 for the metal-rich GCs, and to a combination of accreted and in-situ formation modes at z ~ 5-6 for the metal-poor GCs. An increasingly diverse zoo of compact stellar systems is being discovered, including objects that bridge the gaps between UCDs and faint fuzzies, and between UCDs and compact ellipticals. Many of these have properties pointing to origins as the stripped nuclei of larger galaxies, and a smoking-gun example is presented of an omega Cen-like star cluster embedded in a tidal stream.