Browsing by Author "Zijlstra, AA"

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    A dwarf irregular galaxy at the edge of the local group: Stellar populations and distance of IC5152
    (1999) Zijlstra, AA; Minniti, D
    We have obtained V- and I-band photometry for about 700 stars in the field of the dwarf irregular galaxy IC 5152, previously considered a possible member of the Local Group. Deep VI color-magnitude diagrams are presented of the main body of this galaxy and of a nearby field. We infer a distance by comparison with the VI color-magnitude diagrams of similar galaxies and with theoretical isochrones. The distance modulus to IC 5152 is found to be m - M-o = 26.15 +/- 0.2, adopting E(V - I) = 0. This distance of D = 1.7 Mpc implies that IC 5152 is not a certain member of the Local Group. We also construct a deep optical luminosity function. By comparison with theoretical isochrones, we find a metallicity Z approximate to 0.002, which is lower than previous estimates. Using this metallicity and distance, IC 5152 now fits the metallicity-luminosity relation for dwarf galaxies. The youngest stars in the held studied have an age of similar to 10(7) Gyr, and there is a substantial population of stars with log t > 7.8-8 yr. The central region of IC 5152 is an active site of star formation, as found by comparing our optical photometry with Hubble Space Telescope ultraviolet images. Several candidate globular clusters are found, as well as a candidate for a potential nucleus.
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    Deep Hubble Space Telescope STIS color-magnitude diagrams of the dwarf irregular galaxy WLM
    (2000) Rejkuba, M; Minniti, D; Gregg, MD; Zijlstra, AA; Alonso, MV; Goudfrooij, P
    We have obtained V- and I-band photometry for 1886 stars down to I = 27 and y = 28 in the held of the dwarf irregular galaxy WLM, using deep Space Telescope Imaging Spectrograph (STIS) CL- and LP-band images taken in the parallel mode with the Hubble Space Telescope. The photometry is used to study the horizontal branch identified in WLM. The horizontal branch, extending blueward from the red giant clump, is an unambiguous signature of an old population. We demonstrate that it is possible to reach the horizontal branch of an old population at a distance of similar or equal to 1 Mpc using STIS, with relatively short exposure times. From the VI color-magnitude diagram, we obtain an accurate distance modulus (m - M)(0) = 24.95 +/- 0.13 for WLM by using the Y magnitude of the horizontal branch, and by adopting E(V-I)= 0.03. The implications are that (1) WLM formed stars at high redshift and (2) the old population of WLM can be representative of a protogalactic fragment, related to those accreted to form the Milky Way halo.