Globular clusters (GCs) are tracers of the gravitational potential of their host galaxies. Moreover, their kinematic properties may provide clues for understanding the formation of GC systems and their host galaxies. We use the largest set of GC velocities obtained so far of any elliptical galaxy to revise and extend the previous investigations ) of the dynamics of NGC 1399, the central dominant galaxy of the nearby Fornax cluster of galaxies. The GC velocities are used to study the kinematics, their relation with population properties, and the dark matter halo of NGC 1399. We have obtained 477 new medium-resolution spectra (of these, 292 are spectra from 265 individual GCs, 241 of which are not in the previous data set). with the VLT FORS 2 and Gemini South GMOS multi-object spectrographs. We revise velocities for the old spectra and measure velocities for the new spectra, using the same templates to obtain an homogeneously treated data set. Our entire sample now comprises velocities for almost 700 GCs with projected galactocentric radii between 6 and 100 kpc. In addition, we use velocities of GCs at larger distances published elsewhere. Combining the kinematic data with wide-field photometric Washington data, we study the kinematics of the metal-poor and metal-rich subpopulations. We discuss in detail the velocity dispersions of subsamples and perform spherical Jeans modelling. The most important results are: the red GCs resemble the stellar field population of NGC 1399 in the region of overlap. The blue GCs behave kinematically more erratic. Both subpopulations are kinematically distinct and do not show a smooth transition. It is not possible to find a common dark halo which reproduces simultaneously the properties of both red and blue GCs. Some velocities of blue GCs are only to be explained by orbits with very large apogalactic distances, thus indicating a contamination with GCs which belong to the entire Fornax cluster rather than to NGC 1399. Also, stripped GCs from nearby elliptical galaxies, particularly NGC 1404, may contaminate the blue sample. We argue in favour of a scenario in which the majority of the blue cluster population has been accreted during the assembly of the Fornax cluster. The red cluster population shares the dynamical history of the galaxy itself. Therefore we recommend to use a dark halo based on the red GCs alone. The dark halo which fits best is marginally less massive than the halo quoted previously. The comparison with X-ray analyses is satisfactory in the inner regions, but without showing evidence for a transition from a galaxy to a cluster halo, as suggested by X-ray work.

We present a Washington C and Kron-Cousins R photometric study of the globular cluster system of NGC 1399, the central galaxy of the Fornax cluster. A large areal coverage of 1 square degree around NGC 1399 is achieved with three adjoining fields of the MOSAIC II Imager at the CTIO 4-m telescope. Working on such a large field, we can perform the first indicative determination of the total size of the NGC 1399 globular cluster system. The estimated angular extent, measured from the NGC 1399 centre and up to a limiting radius where the areal density of blue globular clusters falls to 30 per cent of the background level, is 45 ± 5 arcmin, which corresponds to 220−275 kpc at the Fornax distance. The bimodal colour distribution of this globular cluster system, as well as the different radial distribution of blue and red clusters, up to these large distances from the parent galaxy, are confirmed. The azimuthal globular cluster distribution exhibits asymmetries that might be understood in terms of tidal stripping of globulars from NGC 1387, a nearby galaxy. The good agreement between the areal density profile of blue clusters and a projected dark-matter NFW density profile is emphasized.

Abstract. Previous smaller-scale studies of the globular cluster system of NGC 4636, an elliptical galaxy in the southern part of the Virgo cluster, have revealed an unusually rich globular cluster system. We re-investigate the cluster system of NGC 4636 with wide-field Washington photometry. The globular cluster luminosity function can be followed roughly 1 mag beyond the turn-over magnitude found at V = 23.31 ± 0.13 for the blue cluster sub-population. This corresponds to a distance modulus of (m − M) = 31.24 ± 0.17, 0.4 mag larger than the distance determined from surface brightness fluctuations. The high specific frequency is confirmed, yet the exact value remains uncertain because of the uncertain distance: it varies between 5.6 ± 1.2 and 8.9 ± 1.2. The globular cluster system has a clearly bimodal color distribution. The color peak positions show no radial dependence and are in good agreement with the values found for other galaxies studied in the same filter system. However, a luminosity dependence is found: brighter clusters with an "intermediate" color exist. The clusters exhibit a shallow radial distribution within 7 , represented by a power-law with an exponent of −1.4. Within the same radial interval, the galaxy light has a distinctly steeper profile. Because of the difference in the cluster and light distribution the specific frequency increases considerably with radius. At 7 and 9 the density profiles of the red and blue clusters, respectively, change strongly: the powerlaw indices decrease to around −5 and become similar to the galaxy profile. This steep profile indicates that we reach the outer rim of the cluster system at approximately 11 . This interpretation is supported by the fact that in particular the density distribution of the blue cluster population can be well fit by the projection of a truncated power-law model with a core. This feature is seen for the first time in a globular cluster system. While the radial distribution of the cluster and field populations are rather different, this is not true for the ellipticity of the system: the elongation as well as the position angle of the cluster system agree well with the galaxy light. We compare the radial distribution of globular clusters with the light profiles for a sample of elliptical galaxies. The difference observed in NGC 4636 is typical of an elliptical galaxy of this luminosity. The intrinsic specific frequency of the blue population is considerably larger than that of the red one.

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.

Context. Central galaxies in galaxy clusters may be key discriminants in the competition between the cold dark matter (CDM) paradigm and modified Newtonian dynamics (MOND). Aims. We investigate the dark halo of NGC 1399, the central galaxy of the Fornax cluster, out to a galactocentric distance of 80 kpc. Methods. The data base consists of 656 radial velocities of globular clusters obtained with MXU/VLT and GMOS/Gemini, which is the largest sample so far for any galaxy. We performed a Jeans analysis for a non-rotating isotropic model.Results. An NFW halo with the parameters r s = 50 kpc and s = 0.0065 M /pc 3 provides a good description of our data, fitting well to the X-ray mass. More massive halos are also permitted that agree with the mass of the Fornax cluster as derived from galaxy velocities. We compare this halo with the expected MOND models under isotropy and find that additional dark matter on the order of the stellar mass is needed to get agreement. A fully radial infinite globular cluster system would be needed to change this conclusion. Conclusions. Regarding CDM, we cannot draw firm conclusions. To really constrain a cluster wide halo, more data covering a larger radius are necessary. The MOND result appears as a small-scale variant of the finding that MOND in galaxy clusters still needs dark matter.

Context. We present the first dynamical study of the globular cluster system of NGC 4636. It is the southernmost giant elliptical galaxy of the Virgo cluster and is claimed to be extremely dark matter dominated, according to X-ray observations. Aims. Globular clusters are used as dynamical tracers to investigate, by stellar dynamical means, the dark matter content of this galaxy. Methods. Several hundred medium resolution spectra were acquired at the VLT with FORS 2/MXU. We obtained velocities for 174 globular clusters in the radial range 0. 90 < R < 15. 5, or 0.5−9 R e in units of effective radius. Assuming a distance of 15 Mpc, the clusters are found at projected galactocentric distances in the range 4 to 70 kpc, the overwhelming majority within 30 kpc. The measured line-of-sight velocity dispersions are compared to Jeans-models. Results. We find some indication of a rotation of the red (metal-rich) clusters about the minor axis. Out to a radius of 30 kpc, we find a roughly constant projected velocity dispersion for the blue clusters of σ ≈ 200 km s −1 . The red clusters are found to have a distinctly different behavior: at a radius of about 3 , the velocity dispersion drops by ∼50 km s −1 to about 170 km s −1 , which then remains constant out to a radius of 7 . The cause might be the steepening of the number density profile at ∼3 observed for the red clusters. Using only the blue clusters as dynamical tracers, we perform Jeans-analyses for different assumptions of the orbital anisotropy. Enforcing the model dark halos to be of the NFW type, we determine their structural parameters. Depending on the anisotropy and the adopted M/L-values, we find that the dark matter fraction within one effective radius can vary between 20% and 50%, with most a probable range between 20% and 30%. The ambiguity of the velocity dispersion in the outermost bin is a main source of uncertainty. A comparison with cosmological N-body simulations reveals no striking disagreement. Conclusions. Although the dark halo mass still cannot be strongly constrained, NGC 4636 does not seem to be extremely dark matter dominated. The derived circular velocities are also consistent with Modified Newtonian Dynamics.

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.

We present new radial velocities for 289 globular clusters around NGC 4636, the southernmost giant elliptical galaxy of the Virgo cluster. The data were obtained with FORS2/MXU at the Very Large Telescope. Together with data analysed in an earlier study, we now have a sample of 460 globular cluster velocities out to a radius of 12 arcmin (60 kpc) available -one of the largest of its kind. This new data set also provides a much more complete angular coverage. Moreover, we present new kinematical data of the inner stellar population of NGC 4636. We perform an updated Jeans analysis, using both stellar and GC data, to better constrain the dark halo properties. We find a stellar M/L-ratio of 5.8 in the R-band, higher than expected from single stellar population synthesis. We model the dark halo by cored and cuspy analytical halo profiles and consider different anisotropies for the tracer populations. Properties of NFW halos lie well within the expected range of cosmological simulations. Cored halos give central dark matter densities, which are typical for elliptical galaxies of NGC 4636's luminosity. The surface densities of the dark matter halos are higher than those of spiral galaxies. We compare the predictions of modified Newtonian dynamics with the derived halo properties and find satisfactory agreement. NGC 4636 therefore falls onto the baryonic Tully-Fisher relation for spiral galaxies. The comparison with the X-ray mass profile of Johnson et al. (2009, ApJ, 706, 980) reveals satisfactory agreement only if the abundance gradient of hot plasma has been taken into account. This might indicate a general bias towards higher masses for X-ray based mass profiles in all systems, including galaxy clusters, with strong abundance gradients.