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

Abstract

We use a combined imaging and spectroscopic survey of the nearby central cluster galaxy, M87, to assemble a sample of 34 confirmed ultracompact 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 (M V = −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.

A catalog of extended clusters and ultra-compact dwarf galaxies

“…Dabringhausen et al 2008;Evstigneeva et al 2008) is no longer existing on the basis of the larger data-set presented in this paper. This result is consistent with the conclusions of Brodie et al (2011), which were based on a considerably smaller data-set. Figure 10 shows next to the GCs and EOs (black circles) also the effective radii and absolute luminosities of early-type galaxies (open circles).…”

“…EOs were detected in a large number of elliptical galaxies: NGC 5128 (Gomez et al 2006;McLaughlin et al 2008;Chattopadhyay et al 2009;Taylor et al 2010;Mouhcine et al 2010 (Haşegan et al 2005;Evstigneeva et al 2008;Brodie et al 2011;Chies-Santos et al 2011), M 84 (Chies-Santos et al 2011, the central galaxy of the Fornax Cluster NGC 1399 (Richtler et al 2005;Evstigneeva et al 2007;Hilker et al 2007;Evstigneeva et al 2008;Chilingarian et al 2011), NGC 3923 (Norris & Kannappan 2011), NGC 4476 (Haşegan et al 2005), NGC 5846 (Chies-Santos et al 2006), NGC 4696 (Mieske et al 2007), NGC 3311 ), IC 4041, NGC 4889, IC 3998, IC 4030, IC 4041, and NGC 4908 (Chiboucas et al 2011), NGC 4874 (Madrid et al 2010Chiboucas et al 2011), NGC 1132(Madrid 2011), NGC 1275(Penny et al 2012, NGC 4365 (Blom et al 2012), NGC 1316 (Goudfrooij 2012), NGC 1199(Da Rocha et al 2011, and ESO325-G004 (Blakeslee & Barber 2008).…”

Isolated ellipticals and their globular cluster systems

“…UCDs have been found mostly in galaxy groups and clusters (e.g., Mieske et al 2008;Brodie et al 2011), with formation channels mostly appealing to dynamical effects common in high-density environments (e.g., Hilker et al 1999). If spectroscopically confirmed, the UCDs in NGC 3962 would be the first around an isolated elliptical, confirming that the formation of these objects is not restricted to high-density environments, following the finding of one UCD around the field spiral galaxy M 104 (Hau et al 2009).…”

Ultra-diffuse and Ultra-compact Galaxies in the Frontier Fields Cluster Abell 2744

“…These "ultra-compact dwarfs" (UCDs) have characteristics reminiscent of both the nuclei of low-mass galaxies (Georgiev & Böker 2014), and massive GCs, and they may well have a connection to both populations (see, e.g., Mieske et al 2002Mieske et al , 2012Brodie et al 2011;Norris et al 2014;Zhang et al 2015). UCDs seem to occur mostly in dense environments (both near the centers of clusters and near massive galaxies), suggesting that environmental factors (e.g., tidal stripping) drives their formation (e.g., Bekki et al 2003;Pfeffer & Baumgardt 2013).…”