UCDs in the Coma Cluster

TL;DR

This study identifies and confirms a population of Ultra Compact Dwarf galaxies in the Coma cluster core, revealing their clustering around giant galaxies and similarities to globular clusters, supporting a star cluster origin.

Contribution

First spectroscopic confirmation of UCDs in the Coma cluster core, demonstrating their clustering and globular cluster-like properties, and providing insights into their origin.

Findings

27 UCDs confirmed as cluster members

High clustering of UCDs around giant galaxies

UCDs have similar colors and distributions to globular clusters

Abstract

As part of the HST/ACS Coma Cluster Treasury Survey, we have undertaken a Keck/LRIS spectroscopic campaign to determine membership for faint dwarf galaxies. In the process, we discovered a population of Ultra Compact Dwarf galaxies (UCDs) in the core region of the Coma cluster. At the distance of Coma, UCDs are expected to have angular sizes 0.01 < R_e < 0.2 arcsec. With ACS imaging, we can resolve all but the smallest ones with careful fitting. Candidate UCDs were chosen based on magnitude, color, and degree of resolution. We spectroscopically confirm 27 objects as bona fide UCD members of the Coma cluster, a 60% success rate for objects targeted with M_R < -12. We attribute the high success rate in part to the high resolution of HST data and to an apparent large population of UCDs in Coma. We find that the UCDs tend to be strongly clustered around giant galaxies, at least in the core region of the cluster, and have a distribution and colors that are similar to globular clusters. These findings suggest that UCDs are not independent galaxies, but rather have a star cluster origin. This current study provides the dense environment datapoint necessary for understanding the UCD population.