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Register a new userThe Outer Cluster System of NGC 1399: Preliminary Results
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Y. Schuberth
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We present preliminary results of our dynamical study of the outer globular cluster system of NGC 1399, the central galaxy in the Fornax Cluster. About 160 new radial velocities for globular clusters at projected galactocentric distances between 8 and 18 arcminutes indicate that the constant velocity dispersion of 276 km/s (for all clusters) already found for the inner region can be traced out to 80 kpc. We find that the kinematical properties of the blue (metal-poor) and the red (metal-rich) globular cluster subpopulations appear to be different: While the velocity distribution of the red clusters is symmetric with respect to the systemic velocity of NGC 1399, the blue clusters show a somewhat asymmetric distribution, with more velocities above the systemic velocity.
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We present 8 bright globular clusters and/or objects of less familiar nature which we found in the course of scrutinizing the globular cluster system of NGC 1399. These objects are morphologically striking, either by their sizes or by other structural properties. Some of them may be candidates for stripped dwarf galaxy nuclei, emphasizing the possible role of accretion in the NGC 1399 cluster system. They are all highly interesting targets for further deep spectroscopy or HST-imaging. Since these objects have been found within an area of only 42 arcmin**2, we expect many more still to be detected in a full census of the NGC 1399 cluster system.
(Abridged) We use the largest set of globular cluster 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. Our sample now comprises velocities for almost 700 GCs with projected galactocentric radii between 6 and 100 kpc. In addition, we use velocities published by Bergond et al. (2007). We study the kinematics of the metal-poor and metal-rich subpopulations and perform spherical Jeans modelling. The most important results are: The metal-rich (red) GCs resemble the stellar field population of NGC 1399 in the region of overlap. 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 subpopulations. 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. Stripped GCs from nearby elliptical galaxies, particularly NGC 1404, may also contaminate the metal-poor 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 by Richtler et al. (2004). 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 investigate whether the globular clusters (GCs) in the recently published sample of GCs in the Fornax cluster by Bergond and coworkers are indeed intra-cluster objects. We combine the catalogue of radial velocity measurements by Bergond et al. with our CTIO MOSAIC photometry in the Washington system and analyse the relation of metal-poor and metal-rich GCs with their host galaxies. The metal-rich GCs appear to be kinematically associated with their respective host galaxies. The vast majority of the metal-poor GCs found in between the galaxies of the Fornax cluster have velocities which are consistent with them being members of the very extended NGC 1399 GC system. We find that when the sample is restricted to the most accurate velocity measurements, the GC velocity dispersion profile can be described with a mass model derived for the NGC 1399 GC system within 80 kpc. We identify one ``vagrant GC whose radial velocity suggests that it is not bound to any galaxy unless its orbit has a very large apogalactic distance.
We present VLT (FORS1) photometry of the lower main sequence (MS) of the Galactic Globular Cluster (GGC) NGC 6397, for stars located in 2 fields extending from a region near the cluster center out to ~ 10. The obtained CMD shows a narrow MS extending down to V ~ 27 (figure c), much deeper than any previous ground based study and comparable with previous HST photometry (Cool et al. 1996). The comparison between observed MS Luminosity Functions (LFs) derived for 2 annuli at different radial distance from the center of the cluster shows a clear-cut correlation between their slope before reaching the turn-over, and the radial position of the observed fields inside the cluster area: the LFs become flatter with decreasing radius, a trend that is consistent with the interpretation of NGC 6397 as a dynamically relaxed system.
We have obtained spectroscopic redshifts of colour-selected point sources in four wide area VLT-FLAMES fields around the Fornax Cluster giant elliptical galaxy NGC 1399, identifying as cluster members 30 previously unknown faint (-10.5<M_g<-8.8) 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 1399s 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 1399s inner GC system; (iv) their mean heliocentric recessional velocity is between that of NGC 1399s inner GCs and that of the surrounding dwarf galaxies, but their velocity dispersion is significantly lower; (v) bright CSS (M_V<-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.
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