Browsing by Author "Richtler, T"

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    Stromgren photometry in globular clusters: M55 & M22
    (1999) Richter, P; Hilker, M; Richtler, T
    We present Stromgren CCD photometry for the two galactic globular clusters M55 (NGC6809) and M22 (NGC6656).
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    The 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, H
    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 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).
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    The globular cluster system of NGC 4374
    (2004) Gómez, M; Richtler, T
    We study the globular cluster system (GCS) of the giant elliptical NGC 4374 (M 84) in the Virgo cluster using B and R photometry. The colour distribution is bimodal with peaks at B-R=1.11 and B-R=1.36, fitting well to those found in other early-type galaxies. The radial profile of the cluster number density is flatter than the galaxy light. Using the luminosity function we derive a distance modulus of mu=31.61+/-0.2, which within the uncertainty agrees with the distance from surface brightness fluctuations. Blue and red clusters show similar radial concentrations and azimuthal distributions. The total number of clusters is N=1775+/-150, which together with our distance modulus leads to a specific frequency Of S-N=1.6+/-0.3. This value is surprisingly low for a giant elliptical, but resembles the case of merger remnants like NGC 1316, where the low specific frequency is probably caused by the luminosity contribution of an intermediate-age population. A further common property is the high rate of type Ia supernovae which also may indicate the existence of a younger population. However, unlike in the case of NGC 1316, one cannot find any further evidence that NGC 4374 indeed hosts younger populations. The low specific frequency would also fit to a S0 galaxy seen face-on.
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    ω Centauri -: a former nucleus of a dissolved dwarf galaxy?: New evidence from Stromgren photometry
    (2000) Hilker, M; Richtler, T
    CCD 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.