Aims. We analyse the properties of the early-type dwarf galaxy population (M V > −17 mag) in the Hydra I cluster. We investigate the galaxy luminosity function (LF), the colour-magnitude relation (CMR), and the magnitude-surface brightness relation down to M V ∼ −10 mag. Another goal of this study is to find candidates for ultra-compact dwarf galaxies (UCDs) in Hydra I. Methods. Two spectroscopic surveys performed with Magellan I/LDSS2 at Las Campanas Observatory and VLT/VIMOS, as well as deep VLT/FORS1 images in V and I bands, covering the central parts of the cluster, were examined. We identify cluster members by radial velocity measurements and select other cluster galaxy candidates by their morphology and low surface brightness. The candidates' total magnitudes and central surface brightnesses were derived from the analysis of their surface brightness profiles. To determine the faint-end slope of the LF, the galaxy number counts are completeness corrected. Results. We obtain radial velocities for 126 objects and identify 32 cluster members, of which 5 are previously uncatalogued dwarf galaxies. One possible UCD candidate with M V = −13.26 mag is found. Our sample of 100 morphologically selected dwarf galaxies with M V > −17 mag defines a CMR that extends the CMR of the giant cluster galaxies to the magnitude limit of our survey (M V ∼ −10 mag). It matches the relations found for the Local Group (LG) and the Fornax cluster dwarf galaxies almost perfectly. The Hydra I dwarf galaxies also follow a magnitude-surface brightness relation that is very similar to that of the LG dwarf galaxies. Moreover, we observe a continuous relation for dwarf galaxies and giant early-type galaxies when plotting the central surface brightness μ 0 of a Sérsic model vs. the galaxy magnitude. The effective radius is found to be largely independent of the luminosity for M V > −18 mag. It is consistent with a constant value of R e ∼ 0.8 kpc. We present the photometric parameters of the galaxies as the Hydra I Cluster Catalogue (HCC). By fitting a Schechter function to the luminosity distribution, we derive a very flat faint-end slope of the LF (α = −1.13 ± 0.04), whereas fitting a power law for M V > −14 mag gives α = −1.40 ± 0.18. Conclusions. Our findings of a continuous CMR and μ 0 − M V relation for dwarf and giant early-type galaxies suggests that they are the same class of objects. The similarity of those relations to other environments like the LG implies that internal processes could be more important for their global photometric properties than external influences.

Dynamically hot stellar systems, whether star clusters or early-type galaxies, follow welldefined scaling relations over many orders of magnitudes in mass. These Fundamental Plane relations have been subject of several studies, which have been mostly confined to certain types of galaxies and/or star clusters so far. Here, we present a complete picture of hot stellar systems ranging from faint galaxies and star clusters of only a few hundred solar masses up to giant ellipticals (gEs) with 10 12 M , in particular including -for the first time -large samples of compact ellipticals (cEs), ultracompact dwarf galaxies (UCDs), dwarf ellipticals (dEs) of nearby galaxy clusters and Local Group ultrafaint dwarf spheroidals (dSphs). For all those stellar systems we show the effective radius luminosity, effective radius stellar mass and effective mass surface density stellar mass plane. Two clear families of hot stellar systems can be differentiated: the 'galaxian' family, ranging from gEs over ellipticals (Es) and dEs to dSphs, and the 'star cluster' family, comprising globular clusters (GCs), UCDs and nuclear star clusters (NCs). Interestingly, massive Es have a similar size-mass relation as cEs, UCDs and NCs, with a clear common boundary towards minimum sizes, which can be approximated by R eff ≥ 2.24 × 10 −6 M 4/5 pc. No object of either family is located in the 'zone of avoidance' beyond this limit. Even the majority of early-type galaxies at high redshift obeys this relation. The sizes of dEs and dSphs (R eff ∼ 1.0 kpc) as well as GCs (R eff ∼ 3 pc) barely vary with mass over several orders of magnitude. We use the constant galaxy sizes to derive the distances of several local galaxy clusters. The size gap between star clusters and dwarf galaxies gets filled in by low mass, resolving star clusters and the faintest dSphs at the low mass end, and by GCs/UCDs, NCs and cEs in the mass range 10 6 < M < 10 9 M . In the surface density-mass plane the sequences of star clusters and galaxies show the same slope, but are displaced with respect to each other by 10 3 in mass and 10 2 in surface density. Objects that fall in between both sequences include cEs, UCDs, NCs and ultrafaint dSphs. Both, galaxies and star clusters, do not exceed a surface density of eff = 3.17 × 10 10 M −3/5 M pc −2 , causing an orthogonal kink in the galaxy sequence for Es more massive than 10 11 M . The densest stellar systems (within their effective radius) are NCs.

Context. One formation channel discussed for ultra-compact dwarf galaxies (UCDs) is that of massive star clusters, and the other main scenario is that of tidally transformed dwarf galaxies. Aims. We aim at quantifying the specific frequency of UCDs in a range of environments and at relating this to the frequency of star clusters and potential progenitor dwarf galaxies. Are the frequencies of UCDs consistent with being the bright tail of the globular cluster luminosity function (GCLF)? Methods. We propose a definition for the specific frequency of UCDs,is the zero point of the definition, chosen such that the specific frequency of UCDs is the same as those of globular clusters, S N,GC , if UCDs follow a simple extrapolation of the GCLF. Considering UCDs as compact stellar systems with M V < −10.25 mag (mass above ∼2 × 10 6 M ), it is M V,0 = −20 mag. The parameter c w is a correction term to take the dependence of the GCLF width σ on the host galaxy luminosity into account. We apply our definition of S N,UCD to results of spectroscopic UCD searches in the Fornax, Hydra, and Centaurus galaxy clusters, two Hickson compact groups, and the Local Group. This includes a large database of 180 confirmed UCDs in Fornax. Results. We find that the specific frequencies derived for UCDs match those of GCs very well, to within 10-50%. The ratio S N,UCD S N,GC is 1.00 ± 0.44 for the four environments Fornax, Hydra, Centaurus, and Local Group, which have S N,GC values. This good match also holds for individual giant galaxies in Fornax and in the Fornax intracluster-space. The error ranges of the derived UCD specific frequencies in the various environments then imply that not more than ∼50% of UCDs were formed from dwarf galaxies. We show that such a scenario would require 90% of primordial dwarfs in galaxy cluster centers (<100 kpc) to have been stripped of their stars. Conclusions. We conclude that the number counts of UCDs are fully consistent with them being the bright tail of the GC population. From a statistical point of view there is no need to invoke an additional formation channel.

Context. Some galaxy clusters exhibit shallow or even cored dark matter density profiles in their central regions rather than the predicted steep or cuspy profiles, conflicting with the standard understanding of dark matter. NGC 3311 is the central cD galaxy of the Hydra I cluster (Abell 1060). Aims. We use globular clusters around NGC 3311, combined with kinematical data of the galaxy itself, to investigate the dark matter distribution in the central region of Hydra I. Methods. Radial velocities of 118 bright globular clusters, based on VLT/VIMOS mask spectroscopy, are used to calculate velocity dispersions which are well defined out to 100 kpc. NGC 3311 is the most distant galaxy for which this kind of study has been performed. We also determine the velocity dispersions of the stellar component from long-slit spectroscopy of NGC 3311 acquired with VLT/FORS1 out to 20 kpc. We present a new photometric model for NGC 3311, based on deep VLT/FORS1 images in the V-band. We search for a dark halo that, in the context of a spherical Jeans model, can reproduce the kinematical data. We also compare the radial velocity distributions of globular clusters and planetary nebulae. Results. The projected stellar velocity dispersion rises from a central low value of about 185 km s −1 to 350 km s −1 at a radius of 20 kpc. The globular cluster dispersion rises as well from 500 km s −1 at 10 kpc to about 800 km s −1 at 100 kpc, comparable to the velocity dispersion of the cluster galaxies. A dark matter halo with a core (Burkert halo) closely reproduces the velocity dispersions of stars and globular clusters simultaneously under isotropy. The central stellar velocity dispersions predicted by cosmological NFW halos do not agree well with those observed, while the globular clusters allow a wide range of halo parameters. A suspected radial anisotropy of the stellar population found in merger simulations aggravates the disagreement with observations. A slight tangential anisotropy would enable the data to be more accurately reproduced. However, we find discrepancies with previous kinematical data that we cannot resolve, which may indicate a more complicated velocity pattern. Conclusions. Although one cannot conclusively demonstrate that the dark matter halo of NGC 3311 has a core rather than a cusp, a core seems to be most consistent with the present data. A more complete coverage of the velocity field and a more thorough analysis of the anisotropy is required to reach firm conclusions.

Aims. We present a photometric study of the early-type dwarf galaxy population of the Centaurus cluster, aiming at investigating the galaxy luminosity function (LF) and galaxy scaling relations down to the regime of galaxies with M V ∼ −10 mag. Methods. On deep VLT/FORS1 V-and I-band images of the central part of the cluster, we identify cluster dwarf-galaxy candidates using both morphological and surface brightness selection criteria. Photometric and structural parameters of the candidates are derived from analysis of their surface brightness profiles. Fundamental scaling relations, such as the colour-magnitude and the magnitudesurface brightness relation, are used to distinguish the cluster from the background. Results. We find a flat LF with a slope of α = −1.14 ± 0.12 for M V > −14 mag, when fitting a power law to the completenesscorrected galaxy number counts. Applying a Schechter function leads to a consistent result of α ∼ −1.1. When plotting the central surface brightness of a Sérsic model vs. the galaxy magnitude, we find a continuous relation for magnitudes −20 < M V < −10 mag, with only the brightest core galaxies deviating from this relation, in agreement with previous studies of other clusters. Within our Centaurus dwarf galaxy sample we identify three very compact objects. We discuss whether they belong to the class of the so-called compact elliptical galaxies (cEs). In a size-luminosity diagram (R eff vs. M V ) of early-type galaxies from a range of environments, we observe that R eff slowly decreases with decreasing luminosity for −21 < M V < −13 mag and decreases more rapidly at fainter magnitudes. This trend continues to the ultra-faint Local Group dwarf galaxies (M V ∼ −4 mag). Conclusions. The continuous central surface brightness vs. absolute magnitude relation and the smooth relation in the size-luminosity diagram over a wide range of magnitudes are consistent with the interpretation of dwarf galaxies and more massive elliptical galaxies being one family of objects with gradually changing structural properties. The most massive core galaxies and the rare cE galaxies are the only exceptions.

Aims. We performed a large spectroscopic survey of compact, unresolved objects in the core of the Hydra I galaxy cluster (Abell 1060), with the aim of identifying ultra-compact dwarf galaxies (UCDs) and investigating the properties of the globular cluster (GC) system around the central cD galaxy NGC 3311. Methods. We obtained VIMOS medium-resolution spectra of about 1200 candidate objects with apparent magnitudes 18.5 < V < 24.0 mag, covering both the bright end of the GC luminosity function and the luminosity range of all known UCDs. Results. By means of spectroscopic redshift measurements, we identified 118 cluster members, from which 52 are brighter than M V = −11.0 mag, and can therefore be termed UCDs. The brightest UCD in our sample has an absolute magnitude of M V = −13.4 mag (corresponding to a mass of 5 × 10 7 M ) and a half-light radius of 25 pc. This places it among the brightest and most massive UCDs ever discovered. Most of the GCs/UCDs are both spatially and dynamically associated to the central cD galaxy. The overall velocity dispersion of the GCs/UCDs is comparable to what is found for the cluster galaxies. However, when splitting the sample into a bright and a faint part, we observe a lower velocity dispersion for the bright UCDs/GCs than for the fainter objects. At a dividing magnitude of M V = −10.75 mag, the dispersions differ by more than 200 km s −1 and up to 300 km s −1 for objects within 5 around NGC 3311. Conclusions. We interpret these results in the context of different UCD formation channels, and conclude that interaction-driven formation seems to play an important role in the centre of Hydra I.

Studies of dwarf spheroidal (dSph) galaxies with statistically significant sample sizes are still rare beyond the Local Group, since these low surface brightness objects can only be identified with deep imaging data. In galaxy clusters, where they constitute the dominant population in terms of number, they represent the faint end slope of the galaxy luminosity function and provide important insight on the interplay between galaxy mass and environment. In this study we investigate the optical photometric properties of early-type galaxies (dwarf ellipticals (dEs) and dSphs) in the Virgo cluster core region, by analysing their location on the colour magnitude relation (CMR) and the structural scaling relations down to faint magnitudes, and by constructing the luminosity function to compare it with theoretical expectations. Our work is based on deep CFHT V-and I-band data covering several square degrees of the Virgo cluster core that were obtained in 1999 using the CFH12K instrument. We visually select potential cluster members based on morphology and angular size, excluding spiral galaxies. A photometric analysis has been carried out for 295 galaxies, using surface brightness profile shape and colour as further criteria to identify probable background contaminants. 216 galaxies are considered to be certain or probable Virgo cluster members. Our study reveals 77 galaxies not catalogued in the VCC (with 13 of them already found in previous studies) that are very likely Virgo cluster members because they follow the Virgo CMR and exhibit low Sérsic indices. Those galaxies reach M V = −8.7 mag. The CMR shows a clear change in slope from dEs to dSphs, while the scatter of the CMR in the dSph regime does not increase significantly. Our sample might, however, be somewhat biased towards redder colours. The scaling relations given by the dEs appear to be continued by the dSphs indicating a similar origin. The observed change in the CMR slope may mark the point at which gas loss prevented significant metal enrichment. The almost constant scatter around the CMR possibly indicates a short formation period, resulting in similar stellar populations. The luminosity function shows a Schechter function's faint end slope of α = −1.50 ± 0.17, implying a lack of galaxies related to the expected number of low-mass dark matter haloes from theoretical models. Our findings could be explained by suppressed star formation in low-mass dark matter halos or by tidal disruption of dwarfs in the dense core region of the cluster.

Context. In a previous paper, we initiated a search for ultra-compact dwarf galaxies (UCDs) in the Centaurus galaxy cluster, resulting in the discovery of 27 compact objects with −12.2 < M V < −10.9 mag. Our overall survey completeness was 15-20% within 120 kpc projected clustercentric distance. Aims. To constrain the luminosity distribution of the brightest UCDs in Centaurus in a more accurate way, we continue our search by improving substantially our survey completeness specifically for brightnesses of M V < −12 mag (V 0 < 21.3 mag). Methods. Using VIMOS at the VLT, we obtained low resolution spectra of 400 compact objects with 19.3 < V 0 < 21.3 mag (−14 < M V < −12 mag at the Centaurus distance) in the central 25 of the Centaurus cluster, which corresponds to a projected radius of ∼150 kpc. Our survey had complete area coverage within a radius of ∼120 kpc. Results. For 94% of the sources included in the masks, we measure successfully a redshift. Because of incompleteness in the slit assignment, our final completeness in the area surveyed is 52%. Among our targets, we find three new UCDs in the magnitude range −12.2 < M V < −12 mag, hence at the faint limit of our survey. For one of these targets HST WFPC2 imaging is available, yielding a size estimate of r h < ∼ 8−9 pc. At 95% confidence, we can reject the hypothesis that in the area surveyed there are more than 2 massive UCDs with M V < −12.2 mag and r eff < ∼ 70 pc. Hence our survey confirms the extreme rarity of massive UCDs. We find that the radial distributions of Centaurus and Fornax UCDs with respect to their host clusters' centres agree within the 2σ level.