Browsing by Author "Hilker, M"
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- ItemDistance to the Centaurus cluster and its subcomponents from surface brightness fluctuations(2003) Mieske, S; Hilker, MWe present I-band Surface Brightness Fluctuations (SBF) measurements for 15 early type galaxies ( 3 giants, 12 dwarfs) in the central region of the Centaurus cluster, based on deep photometric data in 7 fields obtained with VLT FORS1 and with very good seeing. From the SBF-distances to our sample galaxies we determine the distance of the Centaurus cluster to be 41.3 +/- 2.1 Mpc (33.08 +/- 0.11 mag). This places the Centaurus cluster at about the same distance as the "Great Attractor". We find a distance difference of 0.27 +/- 0.34 mag between the two subcomponents Cen 30 and Cen 45, ruling out that both components are separated by their Hubble flow distance. A distance difference of 0.48 +/- 0.21 mag is found between the central galaxies NGC 4696 (Cen 30) and NGC 4709 (Cen 45) of both components, supported by the different turn-over magnitudes of their respective globular cluster systems. This suggests that Cen 45 is falling into but has not yet reached Cen 30, supporting the idea of a large scale filament along the line of sight towards Centaurus (Churazov et al. 1999). H-0 = 83.0 +/- 8.3 km s(-1) Mpc(-1) is obtained for our Cen 30 sample taking into account the peculiar motion of the Local Group into the direction of the Centaurus cluster. This value of H0 corresponds to a much smaller Hubble flow distortion in the direction of Centaurus than determined by Tonry et al. ( 2000), implying that the GA mass estimate by Tonry et al. may be too high and/or that the Centaurus cluster falls into the GA almost perpendicularly to the line of sight. As our mean single measurement error is very close to the measured distance scatter of the investigated galaxies, we can only derive an upper limit of +/- 10 Mpc radial extension for the Centaurus cluster, corresponding to a five times larger radial than tangential extension. No evidence for an infall pattern into the Great Attractor is found within the uncertainties for the 11 galaxies with measured redshifts.
- ItemExtragalactic globular clusters in the near-infrared - II. The globular clusters systems of NGC 3115 and NGC 4365(2002) Puzia, TH; Zepf, SE; Kissler-Patig, M; Hilker, M; Minniti, D; Goudfrooij, PWe combine near-infrared photometry obtained with the VLT/ISAAC instrument and archival HST/WFPC2 optical images to determine VIK magnitudes and colours of globular clusters in two early-type galaxies, NGC 3115 and NGC 4365. The combination of near-IR and optical photometry provides a way to lift the age-metallicity degeneracy. For NGC 3115, the globular cluster colours reveal two major sub-populations, consistent with previous studies. By comparing the V-I, V-K colours of the NGC 3115 globular clusters with stellar populations models, we find that the colour difference between the two greater than or similar to 10 Gyr old major sub-populations is primarily due to a difference in metallicity. We find Delta[Fe/H] = 1.0 +/- 0.3 dex and the blue (metal-poor) and red (metal-rich) globular cluster sub-populations being coeval within 3 Gyr. In contrast to the NGC 3115 globular clusters, the globular cluster system in NGC 4365 exhibits a more complex age and metallicity structure. We find a significant population of intermediate-age very metal-rich globular clusters along with an old population of both metal-rich and metal-poor clusters. Specifically, we observe a large population of globular clusters with red V K colours but intermediate V I colours, for which all current stellar population models give ages and metallicities in the range similar to2-8 Gyr and similar to0.5 Z(.) -3 Z(.), respectively. After 10 Gyr of passive evolution, the intermediate-age globular clusters in NGC 4365 will have colours which are consistent with the very metal-rich population of globular clusters in giant elliptical galaxies, such as M 87. Our results for both globular cluster systems are consistent with previous age and metallicity studies of the diffuse galactic light. In addition to the major globular cluster populations in NGC 3115 and NGC 4365 we report on the detection of objects with extremely red colours (V-K greater than or similar to3.8 mag), whose nature could not ultimately be revealed with the present data.
- ItemFaint dwarf spheroidals in the Fornax Cluster - A flat luminosity function(2003) Hilker, M; Mieske, S; Infante, LWe have discovered similar or equal to70 very faint dwarf galaxies in the Fornax Cluster. These dSphs candidates follow the same magnitude-surface brightness relation as their counterparts in the Local Group, and even extend it to fainter limits. The faintest dSph candidate in our sample has an absolute magnitude of M-V similar or equal to -8.8 mag and a central surface brightness of mu(V) similar or equal to 27 mag/arcsec(2). There exists a tight color-magnitude relation for the early-type galaxies in Fornax that extends from the giant to the dwarf regime. The faint-end slope of the luminosity function of the early-type dwarfs is flat (alpha similar or equal to -1.1 +/- 0.1), contrary to the results obtained by Kambas et al. (2000).
- ItemRevised Stromgren metallicity calibration for red giants(2000) Hilker, MA new calibration of the Stromgren (b - y), mi diagram in terms of iron abundance of red giants is presented. This calibration is based on a homogeneous sample of giants in the globular clusters omega Centauri, M 22, and M 55 as well as field giants from the list of Anthony-Twarog & Twarog (1998). Towards high metallicities, the new calibration is connected to a previous calibration by Grebel & Richtler (1992), which was unsatisfactory for iron abudances lower than -1.0 dex. The revised calibration is valid for CN-weak/normal red giants in the abundance range of -2.0 <[Fe/H]< 0.0 dex, and a color range of 0.5 < (b - y) < 1.1 mag.
- ItemStromgren photometry in globular clusters: M55 & M22(1999) Richter, P; Hilker, M; Richtler, TWe present Stromgren CCD photometry for the two galactic globular clusters M55 (NGC6809) and M22 (NGC6656).
- ItemThe central region of the Fornax cluster I. A catalog and photometric properties of galaxies in selected CCD fields(1999) Hilker, M; Kissler-Patig, M; Richtler, T; Infante, L; Quintana, HWe 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 square degrees in total. The limiting surface brightness is around 24 mag arcsec(-2), similar to that of Ferguson's (1989) catalog, whereas our limiting total magnitude is around V similar or equal to 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 (Appendix A)(1). The properties and fit parameters of the surface brightness profiles for a sub-sample are presented as a second catalog (Appendix B)(1).
- Itemω Centauri -: a former nucleus of a dissolved dwarf galaxy?: New evidence from Stromgren photometry(2000) Hilker, M; Richtler, TCCD vby Stromgren photometry of a statistically complete sample of red giants and stars in the main sequence turn-off region in omega Centauri has been used to analyse the apparently complex star formation history of this cluster. From the location of stars in the (b - y),m(1) diagram metallicities have been determined. These have been used to estimate ages of different sub-populations in the color-magnitude diagram and to investigate their spatial distributions. We can confirm several earlier findings. The dominating metal-poor population around -1.7 dex is the oldest population found. More metal-rich stars between [Fe/H]=-1.5 and -1.0 dex tend to be 1-3 Gyr younger. These stars are more concentrated towards the cluster center than the metal-poor ones. The most-metal rich stars around -0.7 dex might be up to 6 Gyr younger than the oldest population. They are asymmetrically distributed around the center with an excess of stars towards the South.