A Wide Field Map of Ultra-Compact Dwarfs in the Coma Cluster

TL;DR

This study maps and analyzes ultra-compact dwarfs in the Coma Cluster using Hubble data, revealing their distribution, properties, and possible origins, and highlighting differences from globular clusters.

Contribution

It provides the first extensive catalog of UCDs in Coma, analyzes their spatial distribution, and suggests a bimodal formation scenario based on luminosity and color data.

Findings

Identified 523 UCD candidates in Coma.

UCDs are more centrally concentrated around major ellipticals.

Estimated over 69,000 compact stellar systems in the core of Coma.

Abstract

A dataset of 23,351 globular clusters (GCs) and ultra-compact dwarfs (UCDs) in the Coma cluster of galaxies was built using Hubble Space Telescope Advanced Camera for Surveys data. Based on the standard magnitude cut of $M_V \leq -11$, a total of 523 UCD candidates are found within this dataset of Compact Stellar Systems (CSS). From a color-magnitude diagram (CMD) analysis built using this catalog, we find a clear mass-magnitude relation extending marginally into the UCD parameter space. The luminosity function defined by this dataset, shows an excess of sources at bright magnitudes, suggesting a bimodal formation scenario for UCDs. We estimate the number of UCDs with a different origin than GC to be $N_{UCD} \geq 32 \pm 1$. We derive the total number of CSS within the core (1 Mpc) of Coma to be $N_{CSS} \approx 69,400 \pm 1400$. The radial distribution of UCDs in Coma shows that, like GCs, UCDs agglomerate around three giant ellipticals: NGC 4874, NGC 4889, and IC 4051. We find UCDs are more centrally concentrated around these three ellipticals than GCs. IC 4051 has a satellite population of UCDs similar to NGC 4874 and NGC 4889. We estimate only ~14% of UCDs, inhabit the intracluster space (ICUCD) between galaxies in the region, in comparison to ~24% for GCs (ICGC). We find red (metal-rich) UCDs are more likely located closer to a host galaxy, with blue (metal-poor) UCDs showing a greater dispersion and lower average density in the region.