Intra Cluster Globular Clusters around NGC 1399 in Fornax?

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.Comment: 4 pages, 4 figures, accepted for publication as a Letter in A&

The globular cluster system of NGC 1316 IV. Nature of the star cluster complex SH2

The light of the merger remnant NGC 1316 is dominated by old and intermediate-age stars. The only sign of current star formation in this big galaxy is the HII region SH2, an isolated star cluster complex with a ring-like morphology and an estimated age of 0.1 Gyr at a galactocentric distance of about 35 kpc. A nearby intermediate-age globular cluster, surrounded by weak line emission and a few more young star clusters, is kinematically associated. The origin of this complex is enigmatic. The nebular emission lines permit a metallicity determination which can discriminate between a dwarf galaxy or other possible precursors. We used the Integrated Field Unit of the VIMOS instrument at the Very Large Telescope of the European Southern Observatory to study the morphology, kinematics, and metallicity employing line maps, velocity maps, and line diagnostics of a few characteristic spectra. The line ratios of different spectra vary, indicating highly structured HII regions, but define a locus of uniform metallicity. The strong-line diagnostic diagrams and empirical calibrations point to a nearly solar or even super-solar oxygen abundance. The velocity dispersion of the gas is highest in the region offset from the bright clusters. Star formation may be active on a low level. There is evidence for a large-scale disk-like structure in the region of SH2, which would make the similar radial velocity of the nearby globular cluster easier to understand. The high metallicity does not fit to a dwarf galaxy as progenitor. We favour the scenario of a free-floating gaseous complex having its origin in the merger 2 Gyr ago. Over a long period the densities increased secularly until finally the threshold for star formation was reached. SH2 illustrates how massive star clusters can form outside starbursts and without a considerable field population.Comment: 10 pages, 5 figures, accepted for Astronomy & Astrophysic

The central region of the Fornax cluster -- II. Spectroscopy and radial velocities of member and background galaxies

Radial velocities of 94 galaxies brighter than about V_tot = 20 mag in the direction of the central Fornax cluster have been measured. Except for 8 Fornax members, all galaxies lie in the background. Among the 8 members, there are 5 nucleated dwarf ellipticals that are already listed in the FCC (Ferguson 1989, AJ 98, 367). Two of the 3 ``new'' members are very compact and have surface brightnesses comparable to globular clusters, however their luminosities are in the range of dwarf elliptical nuclei. The measured line indices (especially Mg2, H beta, and iron) of the brighter of the compact objects suggest a solar metallicity, whereas the fainter compact object as well as the dE,Ns have line indices that are similar to those of old metal-poor globular clusters (GCs). However, with these data it is not possible to clearly classify the compact objects either as very bright globular clusters, isolated nuclei of dE,Ns, or even compact ellipticals. A background galaxy cluster at z = 0.11 has been found just behind the center of the Fornax cluster. This explains the excess population of galaxies reported in Paper I. The brightest galaxy of the background cluster lies only 1.1 arcmin south of NGC 1399 and is comparable in absolute luminosity with the central Fornax galaxy itself.Comment: 12 pages, LaTeX2e, uses aa.cls, including 9 PostScript figures; accepted for publication in A&AS, also available at http://www.astro.puc.cl/~mhilker/publication.htm

The extended star formation history of omega Centauri

For the first time, the abundances of a large sample of subgiant and turn-off region stars in omega Centauri have been measured, the data base being medium resolution spectroscopy from FORS2 at the VLT. Absolute iron abundances were derived for about 400 member stars from newly defined line indices with an accuracy of +/-0.15 dex. The abundances range between -2.2<[Fe/H]<-0.7 dex, resembling the large metallicity spread found for red giant branch stars. The combination of the spectroscopic results with the location of the stars in the colour magnitude diagram has been used to estimate ages for the individual stars. Whereas most of the metal-poor stars are consistent with a single old stellar population, stars with abundances higher than [Fe/H]=-1.3 dex are younger. The total age spread in omega Cent is about 3 Gyr. The monotonically increasing age-metallicity relation seems to level off above [Fe/H]=-1.0 dex. Whether the star formation in omega Cen occured continuously or rather episodically has to be shown by combining more accurate abundances with highest quality photometry.Comment: 4 pages, 3 figures, accepted for publication in Astronomy & Astrophysics (Letters

The central region of the Fornax cluster -- I. A catalog and photometric properties of galaxies in selected CCD fields

We present a photometric catalog (based on V and I photometry) of galaxies in the central regions of the Fornax galaxy cluster. Our 11 CCD fields cover 0.17 degrees in total. The limiting surface brightness is around 24 mag arsec^-2, similar to that of Ferguson's (1989, AJ 98, 367) catalog, whereas our limiting total magnitude is around V = 22 mag, about two magnitudes fainter. It is the surface brightness limit, however, that prevents us from detecting the counterparts of the faintest Local Group dwarf spheroidals. The photometric properties of all objects are presented as a catalog. The properties and fit parameters of the surface brightness profiles for a sub-sample are presented as a second catalog (both catalogs are available in electronic form at the CDS). We can only add 4 new dwarf galaxies to Ferguson's catalog. However, we confirm that the dwarf galaxies in Fornax follow a similar surface brightness-magnitude relation as the Local Group dwarfs. They also follow the color (metallicity) - relation seen in other galaxy clusters. A formerly suspected excess of dwarf galaxies surrounding the central cD galaxy NGC 1399 can finally be ruled out. An enhanced density of objects around NGC 1399 can indeed be seen, but it appears displaced with respect to the central galaxy and is identified as a background cluster at z = 0.11 in Paper II of these series, which will discuss spectroscopic results for our sample.Comment: 15 pages, LaTeX2e, uses aa.cls, including 10 PostScript figures, 1 additional gif figure; accepted for publication in A&AS, also available at http://www.astro.puc.cl/~mhilker/publication.htm

The curious case of the companion: evidence for cold accretion onto a dwarf satellite near the isolated elliptical NGC 7796

The isolated elliptical (IE) NGC 7796 is accompanied by an interesting early-type dwarf galaxy, named NGC7796-DW1. It exhibits a tidal tail, very boxy isophotes, and multiple nuclei or regions (A, B, and C) that are bluer than the bulk population of the galaxy, indicating a younger age. These properties are suggestive of a dwarf-dwarf merger remnant. We use the Multi-Unit Spectroscopic Explorer (MUSE) at the VLT to investigate NGC 7796-DW1. We extract characteristic spectra to which we apply the STARLIGHT population synthesis software to obtain ages and metallicities of the various population components of the galaxy. The galaxy's main body is old and metal-poor. A surprising result is the extended line emission in the galaxy, forming a ring-like structure with a projected diameter of 2.2 kpc. The line ratios fall into the regime of HII-regions, although OB-stellar populations cannot be identified by spectral signatures. Nucleus A is a relatively old (7 Gyr or older) and metal-poor super star cluster, most probably the nucleus of the dwarf, now displaced. The star-forming regions B and C show younger and distinctly more metal-rich components. The emission line ratios of regions B and C indicate an almost solar oxygen abundance, if compared with radiation models of HII regions. NGC7796-DW1 occupies a particular role in the group of transition-type galaxies with respect to its origin and current evolutionary state, being the companion of an IE. The dwarf-dwarf merger scenario is excluded because of the missing metal-rich merger component. A viable alternative is gas accretion from a reservoir of cold, metal-rich gas. NGC7796 has to provide this gas within its X-ray bright halo. As illustrated by NGC7796-DW1, cold accretion may be a general solution to the problem of extended star formation histories in transition dwarf galaxies. (abridged)Comment: comments: 13 pages, 8 figures, accepted for publication in Astronomy & Astrophysic