M32 Analogs? A Population of Massive Ultra-compact Dwarf and Compact Elliptical Galaxies in Intermediate-redshift Clusters

Abstract

We report the discovery of relatively massive, M32-like ultra compact dwarf (UCD) and compact elliptical (CE) galaxy candidates in 0.2 < z < 0.6 massive galaxy clusters imaged by the Cluster Lensing And Supernova survey with Hubble (CLASH) survey. Examining the nearly unresolved objects in the survey, we identify a sample of compact objects concentrated around the cluster central galaxies with colors similar to cluster red sequence galaxies. Their colors and magnitudes suggest stellar masses around 10 9 M . More than half of these galaxies have half-light radii smaller than 200 pc, falling into the category of massive UCDs and CEs, with properties similar to M32. The properties are consistent with a tidal stripping origin, but we cannot rule out the possibility that they are earlyformed compact objects trapped in massive dark matter halos. The 17 CLASH clusters studied in this work on average contain 2.7 of these objects in their central 0.3 Mpc and 0.6 in their central 50 kpc. Our study demonstrates the possibility of statistically characterizing UCDs/CEs with a large set of uniform imaging survey data.

Two channels for the formation of compact dwarf galaxies in clusters of galaxies

“…This result is consistent with the preliminary results by Zhang & Bell (2017). They observed 17 clusters of galaxies and found 45 compact dwarfs inside 300 Mpc and 10 inside 50 kpc from the brightest galaxies in the clusters.…”

“…Apart from UCDs, in recent years additional M32-like compact ellipticals (cEs) have been found as well (Mieske et al 2005;Chilingarian et al 2007;Chilingarian & Mamon 2008;Zhang & Bell 2017), additionally bridging the gap between GCs, UCDs and dEs. Most of M32-like cEs were identified in the vicinity of the most massive galaxies inside the clusters.…”

The formation of ultra compact dwarf galaxies and massive globular clusters

“…The presence of UCDs has been reported also in other galaxy clusters, for example Virgo (Drinkwater et al 2004), Coma (Price et al 2009), Centaurus (Mieske et al 2007), Hydra I (Misgeld et al 2011), and Perseus (Penny et al 2012), or in massive intermediate-redshift clusters (e.g. Zhang & Bell 2017). The existence of systems with properties of UCDs was discussed already by Kroupa (1998) based on the observations of star cluster complexes in the Antennae galaxies.…”

Fluctuation of the background sky in the Hubble Extremely Deep Field (XDF) and its origin

“…Since the angular distance has a maximum at z ∼1.6, where 0.1 arcsec corresponds to 860 pc, the physical size of r e must be smaller than 172 pc. This size corresponds to the low end of the size range of ordinary dwarf spheroids (Zhang and Bell 2017).…”