New Ultra-Compact Dwarf Galaxies in Clusters

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How do ultra-compact dwarf galaxies (UCDs) — galaxies that are especially small and dense — form and evolve? Scientists have recently examined distant galaxy clusters, searching for more UCDs to help us answer this question.

Origins of Dwarfs

In recent years we have discovered a growing sample of small, very dense galaxies. Galaxies that are tens to hundreds of light-years across, with masses between a million and a billion solar masses, fall into category of “ultra-compact dwarfs” (UCDs).

UCD candidate

An example of an unresolved compact object from the authors’ survey that is likely an ultra-compact dwarf galaxy. [Adapted from Zhang & Bell 2017]

How do these dense and compact galaxies form? Two possibilities are commonly suggested:

  1. An initially larger galaxy was tidally stripped during interactions with other galaxies in a cluster, leaving behind only its small, dense core as a UCD.
  2. UCDs formed as compact galaxies at very early cosmic times. The ones living in a massive dark matter halo may have been able to remain compact over time, evolving into the objects we see today.

To better understand which of these formation scenarios applies to which galaxies, we need a larger sample size! Our census of UCDs is fairly limited — and because they are small and dim, most of the ones we’ve discovered are in the nearby universe. To build a good sample, we need to find UCDs at higher redshifts as well.

A New Sample

In a recent study, two scientists from University of Michigan have demonstrated how we might find more UCDs. Yuanyuan Zhang (also affiliated with Fermilab) and Eric Bell used the Cluster Lensing and Supernova Survey with Hubble (CLASH) to search 17 galaxy clusters at intermediate redshifts of 0.2 < z < 0.6, looking for unresolved objects that might be UCDs.

UCD distribution

The mass and size distributions of the UCD candidates reported in this study, in the context of previously known nuclear star clusters, globular clusters (GCs), UCDs, compact elliptical galaxies (cEs), and dwarf galaxies. [Zhang & Bell 2017]

Zhang and Bell discovered a sample of compact objects — grouped around the central galaxies of the clusters — that are consistent with ultra-compact galaxies. The inferred sizes (many around 600 light-years in radius) and masses (roughly one billion solar masses) of these objects suggest that this sample may contain some of the densest UCDs discovered to date.

The properties of this new set of UCD candidates aren’t enough to distinguish between formation scenarios yet, but the authors argue that if we find more such galaxies, we will be able to use the statistics of their spatial and color distributions to determine how they were formed.

Zhang and Bell estimate that the 17 CLASH clusters studied in this work each contain an average of 2.7 of these objects in the central million light-years of the cluster. The authors’ work here suggests that searching wide-field survey data for similar discoveries is a plausible way to increase our sample of UCDs. This will allow us to statistically characterize these dense, compact galaxies and better understand their origins.

Citation

Yuanyuan Zhang and Eric F. Bell 2017 ApJL 835 L2. doi:10.3847/2041-8213/835/1/L2

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