The Relationships Among Compact Stellar Systems: A Fresh View of Ultra Compact Dwarfs

TL;DR

This study expands the sample of ultra compact dwarfs (UCDs) around M87, revealing their properties, origins, and relationships with other stellar systems, suggesting most UCDs are stripped galaxy nuclei with diverse formation histories.

Contribution

The paper provides the largest M87 UCD sample to date, analyzes their properties, and offers new insights into their origins as stripped nuclei or star clusters, challenging previous size-luminosity correlations.

Findings

No correlation between size and luminosity in UCDs.

Most UCDs are a distinct population from globular clusters.

UCDs are likely stripped nuclei, with color-magnitude relations linked to galaxy formation processes.

Abstract

We use a combined imaging and spectroscopic survey of the nearby central cluster galaxy, M87, to assemble a sample of 34 confirmed ultra compact dwarfs (UCDs) with half-light radii of >~ 10 pc measured from Hubble Space Telescope images. This doubles the existing sample in M87, making it the largest such sample for any galaxy, while extending the detection of UCDs to unprecedentedly low luminosities (MV = -9). With this expanded sample, we find no correlation between size and luminosity, in contrast to previous suggestions, and no general correlation between size and galactocentric distance. We explore the relationships between UCDs, less luminous extended clusters (including faint fuzzies), globular clusters (GCs), as well as early-type galaxies and their nuclei, assembling an extensive new catalog of sizes and luminosities for stellar systems. Most of the M87 UCDs follow a tight color-magnitude relation, offset from the metal-poor GCs. This, along with kinematical differences, demonstrates that most UCDs are a distinct population from normal GCs, and not simply a continuation to larger sizes and higher luminosities. The UCD color-magnitude trend couples closely with that for Virgo dwarf elliptical nuclei. We conclude that the M87 UCDs are predominantly stripped nuclei. The brightest and reddest UCDs may be the remnant nuclei of more massive galaxies while a subset of the faintest UCDs may be tidally limited and related to more compact star clusters. In the broader context of galaxy assembly, blue UCDs may trace halo build-up by accretion of low-mass satellites, while red UCDs may be markers of metal-rich bulge formation in larger galaxies.