Browsing by Author "Rejkuba, M"

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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.
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    Deep VLT search for globular clusters in NGC 5128: Color-magnitude diagrams and globular cluster luminosity function
    (2001) Rejkuba, M
    At the distance of NGC 5128 (3.6 +/- 0.2 Mpc) it is possible to resolve globular clusters with high resolution imaging from the ground, thus allowing the globular cluster candidate selection primarily through their morphological properties. I report the discovery of 71 globular clusters in NGC 5128 on VLT UT1-FORS1 images, including the faintest members (M-V similar to -5) known to date in this galaxy as well as in 6 previously-known clusters. U- and V-band photometry has been measured for all the candidates and the luminosity function, spanning -10.1 < M-V < -4.9 and -9.3 < M-U < -3.3, constructed. These are the deepest globular cluster luminosity functions in an elliptical galaxy determined so far. The Kolmogorov-Smirnov statistics show that the difference between the globular cluster luminosity functions of NGC 5128 and the MW is not larger than the difference between the ones of M 31 and the MW. The (U - V)(0) color histogram shows a bimodal distribution. For 23 globular clusters I obtained K-band images with SOF1 at the NTT in La Silla. Their positions in the (U - V) vs. (V - K) color-color diagram indicate that they are indeed old globular clusters. Assuming that the globular clusters in NGC 5128 span a similar age range as the ones in the Milky Way and adopting a linear fit between the metallicity and (U - V)(0) color, the metal-rich clusters peak at [Fe/H] = -0.6 dex and the metal-poor ones peak at [Fe/H] = -1.7 dex.
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    Optical counterparts of X-ray point sources observed by Chandra in NGC 5128
    (2004) Minniti, D; Rejkuba, M; Funes, JG; Akiyama, S
    Very Large Telescope images in BVI are used to identify the optical counterparts to bright Chandra X-ray points sources discovered by Kraft and coworkers. Of a total of 111 X-ray point sources with L-X > 2 x 10(36) ergs s(-1) present in a 56 arcmin(2) field centered on this galaxy, 58 have optical counterparts. On the basis of the sizes, optical magnitudes, and colors, 20 new globular cluster counterparts of X-ray sources are identified, and three are identified on the basis of their sizes. This brings the total number of globular cluster X-ray sources in this galaxy to 33 and establishes that 30% of the X-ray point sources in NGC 5128 are associated with globular clusters. These X-ray globular clusters occupy the brightest end of the globular cluster luminosity function, indicating that bright low-mass X-ray binaries are preferentially found in massive clusters. Most of the globular clusters with X-ray sources have red colors, with 1.0 < V - I < 1.5, indicating that low-mass X-ray binaries are preferentially formed in metal-rich clusters. The NGC 5128 X-ray globular cluster sources are brighter compared with the Milky Way sources: there are 24 globular clusters with X-ray sources of L-X > 10(37) ergs s(-1). There is, however, no globular cluster X-ray source in NGC 5128 as bright as expected for an accreting black hole. In addition, 31 optical counterparts of X-ray point sources that are not associated with globular clusters are identified. Finally, 53 X-ray point sources (48% of the population) do not have any optical counterparts down to the faintest magnitude limits (B = 25).
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    Stellar populations in NGC 5128 with the VLT: Evidence for recent star formation
    (2001) Rejkuba, M; Minniti, D; Silva, DR; Bedding, TR
    We resolve stars of the nearest giant elliptical galaxy NGC 5128 using VLT with FORS1 and ISAAC. We construct deep U, V and K-s color-magnitude and color-color diagrams in two different halo fields (in the halo and in the north-eastern diffuse shell). In the outer, shell field, at similar to 14 kpc from the center of the galaxy, there is a significant recent star formation with stars as young as 10 Myr, approximately aligned with the prominent radio and X-ray jet from the nucleus of the host AGN. Ionized gas filaments are evident in ultraviolet images near the area where neutral Hi and CO molecular gas was previously observed. The underlying stellar population of the halo of the giant elliptical is predominantly old with a very broad metallicity distribution. The presence of an extended giant branch reaching M-bol = -5 mag suggests the existence of a significant intermediate-age AGB population in the halo of this galaxy.
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    Tip of the red giant branch distance for the Sculptor group dwarf ESO 540-032
    (2001) Jerjen, H; Rejkuba, M
    We present the first VI CCD photometry for the Sculptor group galaxy ESO 540-032 obtained at the Very Large Telescope UT1+FORS1. The (I, V - I) colour-magnitude diagram indicates that this intermediate-type dwarf galaxy is dominated by old, metal-poor ([Fe/H] approximate to -1.7 dex) stars, with a small population of slightly more metal-rich ([Fe/H] approximate to -1.3 dex), young (age 150-500 Myr) stars. A discontinuity in the I-band luminosity function is detected at I-0 = 23.44 +/-0.09 mag. Interpreting this feature as the tip of the red giant branch and adopting M-I = -4.20 +/-0.10 mag for its absolute magnitude, we have determined a Population II distance modulus of (m - M)(0) = 27.64 +/-0.14 mag (3.4 +/-0.2 Mpc). This distance confirms ESO 540-032 as a member of the nearby Sculptor group but is significantly larger than a previously reported value based on the Surface Brightness Fluctuation (SBF) method. The results from stellar population synthesis models suggest that the application of the SBF technique on dwarf galaxies with mixed morphology requires a detailed knowledge of the underlying stellar composition and thus offers no advantage over a direct distance measurement using the tip of the red giant branch as distance indicator. We produce the surface brightness profiles for ESO 540-032 and derive the photometric and structural parameters. The global properties follow closely the relations between metallicity and both absolute magnitude and central surface brightness defined by dwarf elliptical galaxies in the Local Group. Finally, we identify and discuss a non-stellar object near the galaxy center which may resemble a globular cluster.