We present new imaging and spectroscopic observations of six ultracompact dwarf (UCD) galaxies in the Virgo Cluster, along with reanalyzed data for five Fornax Cluster UCDs. These are the most luminous UCDs: À14 mag < M V < À12 mag. Our Hubble Space Telescope imaging shows that most of the UCDs have shallow or steep cusps in their cores; only one UCD has a flat ''King'' core. None of the UCDs show tidal cutoffs down to our limiting surface brightness. Spectroscopic analysis shows that Virgo UCDs are old (older than 8 Gyr) and have metallicities in the range from ½Z /H ¼ À1:35 to +0.35 dex. Five Virgo UCDs have supersolar [/Fe] abundance ratios, and one Virgo UCD has a solar abundance ratio. The supersolar [/Fe] abundances are typical of old stellar populations found in globular clusters and elliptical galaxies. We find that Virgo UCDs have structural and dynamical properties similar to Fornax UCDs. The Virgo and Fornax UCDs all have masses %(2Y9) ; 10 7 M and mass-to-light ratios %(3Y5) M /L ;V . The dynamical mass-to-light ratios for Virgo UCDs are consistent with simple stellar population model predictions: the Virgo UCDs do not require dark matter to explain their mass-to-light ratios. We conclude that the internal properties of Virgo UCDs are consistent with them being the high-mass/high-luminosity extreme of known globular cluster populations. We refrain from any firm conclusions on Fornax UCD origins until accurate age, metallicity, and -abundance estimates are obtained for them. Some of our results, notably the fundamental plane projections, are consistent with the formation of UCDs by the simple removal of the halo from the nuclei of nucleated dwarf galaxies. However, the ages, metallicities, and abundances for Virgo UCDs are not consistent with this simple stripping model. It might be consistent with more sophisticated models of the stripping process that include the effects of gas removal on the chemical evolution of the nuclei.

Aims. We determine masses and mass-to-light ratios of five ultra-compact dwarf galaxies (UCDs) and one dwarf elliptical nucleus in the Fornax cluster from high resolution spectroscopy. We examine whether they are consistent with pure stellar populations or whether dark matter is needed to explain their masses. Methods. Velocity dispersions were derived from selected wavelength regions using a direct-fitting method. To estimate the masses of the UCDs a new modelling program has been developed that allows a choice of different representations of the surface brightness profile (i.e. Nuker, Sersic or King laws) and corrects the observed velocity dispersions for observational parameters (i.e. seeing, slit size). The derived dynamical masses are compared to those expected from stellar population models. Results. The observed velocity dispersions range between 22 and 30 km s −1 . The resulting masses are between 1.8 and 9.5 × 10 7 M . These, as well as the central and global projected velocity dispersions, were derived from the generalized King model which turned out to give the most stable results. The masses of two UCDs, that are best fitted by a two-component profile, were derived from a combined King+Sersic model. The mass-to-light ratios of the Fornax UCDs range between 3 and 5 (M/L V ) . The M/L V ratio of the dwarf elliptical nucleus is 2.5. These values are compatible with predictions from stellar population models. Within 1−2 half-mass radii dark matter is not dominating UCDs and the nucleus. An increasing dark matter contribution towards larger radii can not be ruled out with the present data. The M/L V ratios of some UCDs suggest they have intermediate age stellar populations. Conclusions. We show that the mass-to-light ratios of UCDs in Fornax are consistent with those expected for pure stellar populations. Thus UCDs seem to be the result of cluster formation processes within galaxies rather than being compact dark matter dominated substructures themselves. Whether UCDs gained their mass in super-star cluster complexes of mergers or in nuclear star cluster formation processes remains an open question. It appears, however, clear that star clusters more massive than about 5 × 10 6 M exhibit a more complex formation history than the less massive "ordinary" globular clusters.

We present a detailed analysis of high-resolution two-band Hubble Space Telescope Advanced Camera for Surveys 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 stripped ("threshed") early-type dwarf galaxies-by direct 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 in particular cannot 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 (their position in the color-magnitude and luminosity-size diagrams) 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 outward: mean ∆(F 606W − F 814W ) = 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 (>99.9% significance) difference in the sizes of UCDs and early-type galaxy nuclei: the effective radii of UCDs are 2.2 +0.2 −0.1 times larger than those of early-type galaxy nuclei at the same luminosity. This result suggests that an important test can be made of the threshing hypothesis by simulating the process and predicting what size increase is expected.

All the previously cataloged ultracompact dwarf (UCD) galaxies in the Fornax and Virgo clusters have 17.5 < b J < 20. Using the 2dF spectrograph on the Anglo-Australian Telescope, we have carried out a search for fainter UCDs in the Fornax Cluster. In the magnitude interval 19.5 < b J < 21.5, we have found 54 additional compact cluster members within a projected radius of 0.• 9 (320 kpc) of the cluster center, all of which meet our selection and observational criteria to be UCDs. These newly identified objects, however, overlap in luminosity and spatial distribution with objects classified as globular clusters (GCs) belonging to the central cluster galaxy NGC 1399; in fact, about half of the objects in our sample are included in recent catalogs of NGC 1399/Fornax GCs. The numbers, luminosity function, and spatial distributions of our compact object sample are consistent with being the bright tail of the Fornax cluster-wide GC population. Yet, our present larger sample of intergalactic compact objects forms a dynamically distinct population from both the NGC1399 GCs and the nucleated dwarf ellipticals in Fornax. This supports the interpretation that the UCDs, which populate the bright tail of the GC luminosity function, are, in some respects, a separate class of objects, at least to the extent that they have experienced a distinct dynamical history and origin, which differs from the bulk of the NGC1399 GCs. Correcting for our spectroscopic incompleteness, we estimate that there are ∼105 ± 13 of these brighter compact cluster objects down to b J < 21.5 in the central region of the Fornax, and hence these UCDs/globulars outnumber other galaxy types in this space. The differences in their dynamics and distribution compared to dwarf ellipticals (dEs) may be consistent with a threshing or tidal destruction origin, if they have come from a subpopulation of dE galaxies on initial orbits that rendered them susceptible to such processes.

We have redefined group membership of six southern galaxy groups in the local universe (mean cz < 2000   km   s−1) based on new redshift measurements from our recently acquired Anglo‐Australian Telescope 2dF spectra. For each group, we investigate member galaxy kinematics, substructure, luminosity functions and luminosity‐weighted dynamics. Our calculations confirm that the group sizes, virial masses and luminosities cover the range expected for galaxy groups, except that the luminosity of NGC 4038 is boosted by the central starburst merger pair. We find that a combination of kinematical, substructural and dynamical techniques can reliably distinguish loose, unvirialized groups from compact, dynamically relaxed groups. Applying these techniques, we find that Dorado, NGC 4038 and NGC 4697 are unvirialized, whereas NGC 681, NGC 1400 and NGC 5084 are dynamically relaxed.

Using imaging from the Hubble Space Telescope, we derive surface brightness profiles for ultra-compact dwarfs in the Fornax cluster and for the nuclei of dwarf elliptical galaxies in the Virgo cluster. Ultracompact dwarfs are more extended and have higher surface brightnesses than typical dwarf nuclei, while the luminosities, colors, and sizes of the nuclei are closer to those of Galactic globular clusters. This calls into question the production of ultra-compact dwarfs via "threshing" whereby the lower surface brightness envelope of a dwarf elliptical is removed by tidal processes, leaving behind a bare nucleus. Threshing may still be a viable model if the relatively bright Fornax ultra compact dwarfs considered here are descended from dwarf ellipticals whose nuclei are at the upper end of their luminosity and size distributions.

We have obtained spectroscopic redshifts of colour‐selected point sources in four wide area VLT‐FLAMES (Very Large Telescope‐Fibre Large Array Multi Element Spectrograph) fields around the Fornax cluster giant elliptical galaxy NGC 1399, identifying as cluster members 27 previously unknown faint compact stellar systems (CSS), and improving redshift accuracy for 23 previously catalogued CSS. By amalgamating our results with CSS from previous 2dF observations and excluding CSS dynamically associated with prominent (non‐dwarf) galaxies surrounding NGC 1399, we have isolated 80 ‘unbound’ systems that are either part of NGC  1399's globular cluster (GC) system or intracluster GCs. For these unbound systems, we find (i) they are mostly located off the main stellar locus in colour–colour space; (ii) their projected distribution about NGC  1399 is anisotropic, following the Fornax cluster galaxy distribution, and there is weak evidence for group rotation about NGC  1399; (iii) their completeness‐adjusted radial surface density profile has a slope similar to that of NGC  1399's inner GC system; (iv) their mean heliocentric recessional velocity is between that of NGC  1399's inner GCs and that of the surrounding dwarf galaxies, but their velocity dispersion is significantly lower; (v) bright CSS (MV < −11) are slightly redder than the fainter systems, suggesting they have higher metallicity; (vi) CSS show no significant trend in g′−i′ colour index with radial distance from NGC  1399.

We present the results of a search for ultracompact dwarf galaxies (UCDs) in six different galaxy groups: Dorado, NGC 1400, 0681, 4038, 4697 and 5084. We searched in the apparent magnitude range 17.5 b J 20.5 (except NGC 5084: 19.2 b J 21.0). We found one definite plus two possible UCD candidates in the Dorado group and two possible UCD candidates in the NGC 1400 group. No UCDs were found in the other groups. We compared these results with predicted luminosities of UCDs in the groups according to the hypothesis that UCDs are globular clusters formed in galaxies. The theoretical predictions broadly agree with the observational results, but deeper surveys are needed to fully test the predictions.