Structural Properties of Ultra-Compact Dwarf Galaxies in the Fornax and Virgo Clusters

Abstract

We present a detailed analysis of two-band HST/ACS imaging of 21 ultra-compact dwarf (UCD) galaxies in the Virgo and Fornax Clusters. The aim of this work is to test two formation hypotheses for UCDs--whether they are bright globular clusters (GCs) or "threshed'' early-type dwarf galaxies--by comparison of UCD structural parameters and colors with GCs and galaxy nuclei. We find that the UCD surface brightness profiles can be described by a range of models and that the luminous UCDs can not be described by standard King models with tidal cutoffs as they have extended outer halos. This is not expected from traditional King models of GCs, but is consistent with recent results for massive GCs. The total luminosities, colors and sizes of the UCDs are consistent with them being either luminous GCs or threshed nuclei of both early-type and late-type galaxies (not just early-type dwarfs). For the most luminous UCDs we estimate color gradients over a limited range of radius. These are systematically positive in the sense of getting redder outwards: mean Delta(F606W-F814W)=0.14 mag per 100 pc with rms=0.06 mag per 100 pc. The positive gradients found in the bright UCDs are consistent with them being either bright GCs or threshed early-type dwarf galaxies (except VUCD3). In contrast to the above results we find a very significant difference in the sizes of UCDs and early-type galaxy nuclei: the effective radii of UCDs are 2.2 times larger than those of early-type galaxy nuclei at the same luminosity. This result suggests an important test can be made of the threshing hypothesis by simulating the process and predicting what size increase is expected.

29 pages, 16 figures, accepted for publication in The Astronomical Journal

References in corpus (12)

• The ACS Virgo Cluster Survey. VIII. The Nuclei of Early-Type Galaxies
• The ACS Survey of Galactic Globular Clusters. III. The Double Subgiant Branch of NGC 1851
• The ACS Fornax Cluster Survey. I. Introduction to the Survey and Data Reduction Procedures
• The influence of residual gas expulsion on the evolution of the Galactic globular cluster system and the origin of the Population II halo
• Structural parameters for globular clusters in M31 and generalizations for the fundamental plane
• Deep FORS1 observations of the double Main Sequence of omega Centauri
• Dynamical masses of ultra-compact dwarf galaxies in Fornax
• Structural parameters for globular clusters in NGC 5128. III. ACS surface-brightness profiles and model fits
• A search for ultra-compact dwarf galaxies in the Centaurus galaxy cluster
• Searches for Ultra-Compact Dwarf Galaxies in Galaxy Groups
• UCD Candidates in the Hydra Cluster
• The Millennium Galaxy Catalogue: a census of local compact galaxies

Cited by in corpus (32)

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• Globular Cluster Systems in Giant Ellipticals: the Mass/Metallicity Relation
• Uniting Old Stellar Systems: From Globular Clusters to Giant Ellipticals
• The HST/ACS Coma Cluster Survey. X. Nuclear star clusters in low-mass early-type galaxies: scaling relations
• Contribution of stripped nuclear clusters to globular cluster and ultra-compact dwarf galaxy populations
• The Next Generation Virgo Cluster Survey. VI. The Kinematics of Ultra-compact Dwarfs and Globular Clusters in M87
• The Globular Cluster Systems in the Coma Ellipticals. IV: WFPC2 Photometry for Five Giant Ellipticals
• An Ultra Compact Dwarf around the Sombrero galaxy (M104): the Nearest Massive UCD
• The formation of UCDs and massive GCs: Quasar-like objects for testing for a variable stellar initial mass function (IMF)
• An evolutionary continuum from nucleated dwarf galaxies to star clusters
• How elevated is the dynamical-to-stellar mass ratio of the ultra-compact dwarf S999?
• The SLUGGS Survey: HST/ACS Mosaic Imaging of the NGC 3115 Globular Cluster System
• Formation of ultra-compact dwarf galaxies: tests of the galaxy threshing scenario in Fornax
• A metal-poor ultra compact dwarf galaxy at a kiloparsec distance from the low-mass elliptical galaxy FCC47
• Ultra-Compact Dwarfs around NGC 3268
• Ultra-compact dwarfs beyond the centre of the Fornax galaxy cluster: Hints of UCD formation in low-density environments
• A consistency-test for determining whether ultra-compact dwarf galaxies could be the remnant nuclei of threshed galaxies
• Ultra-Compact Dwarf Candidates Near the Lensing Galaxy in Abell S0740
• Compact stellar systems in the Fornax cluster: a UV perspective
• Contribution of stripped nuclei to the ultracompact dwarf galaxy population in the Virgo Cluster
• The Fornax Cluster VLT Spectroscopic Survey IV -- Cold kinematical substructures in the Fornax core from COSTA
• Testing the tidal stripping scenario of ultra-compact dwarf galaxy formation by using internal properties
• M32 Analogs? A Population of Massive Ultra Compact Dwarf and Compact Elliptical Galaxies in intermediate redshift CLASH Clusters
• Distribution of star formation rates during the rapid assembly of NGC 1399 as deduced from its globular cluster system
• Blueshifted galaxies in the Virgo Cluster
• The contribution of supermassive black holes in stripped nuclei to the supermassive black hole population of UCDs and galaxy clusters
• The creation of a massive UCD by tidal threshing from NGC 936
• Dwarf galaxies in the MATLAS Survey: Hubble Space Telescope observations of nuclear star clusters
• VEGAS-SSS: An intra-group component in the globular cluster system of NGC 5018 group of galaxies using VST data
• Observational dynamics of low-mass stellar systems