Browsing by Author "Goudfrooij, P"

Now showing 1 - 5 of 5
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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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    Dissecting the extended main-sequence turn-off of the young star cluster NGC 1850
    (2017) Correnti, M; Goudfrooij, P; Bellini, A; Kalirai, J; Puzia, Thomas H.
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    Extragalactic 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, P
    We 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.
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    Hubble Space Telescope observations of globular cluster systems along the Hubble sequence of spiral galaxies
    (2003) Goudfrooij, P; Strader, J; Brenneman, L; Kissler-Patig, M; Minniti, D; Huizinga, JE
    We have studied the globular cluster systems of seven giant, edge-on spiral galaxies using Hubble Space Telescope imaging in V and I . The galaxy sample covers the Hubble types Sa to Sc, allowing us to study the variation of the properties of globular cluster systems along the Hubble sequence. The photometry reaches similar to1.5 mag beyond the turn-over magnitude of the globular cluster luminosity function for each galaxy. Specific frequencies of globular clusters (S-N values) were evaluated by comparing the numbers of globular clusters found in our WFPC2 pointings with those in our Milky Way that would be detected in the same spatial region if placed at the distance of the target galaxies. Results from this method were found to be consistent with the more commonly used method of constructing radial distribution functions of globular clusters. The S-N values of spirals with B /T less than or similar to 0.3 (i.e. spirals with a Hubble type later than about Sb) are consistent with a value of S-N = 0.55 +/- 0.25. We suggest that this population of globular clusters represents a 'universal', old halo population that is present around each galaxy. Most galaxies in our sample have S-N values that are consistent with a scenario in which globular cluster systems are made up of (i) the aforementioned halo population plus (ii) a population that is associated with bulges, which grows approximately linearly with the mass of the bulge. Such scenarios include the 'merger scenario' for the formation of elliptical galaxies as well as the 'multi-phase collapse' scenario, but it seems inconsistent with the 'secular evolution' scenario of Pfenniger & Norman, in which bulges are formed from disc stars by means of the redistribution of angular momentum through bar instabilities and/or minor perturbations. However, there is one bulge-dominated spiral galaxy in our sample (NGC 7814) with a low S-N value that is consistent with those of the latest-type spirals. This means that the 'secular evolution' scenario can still be viable for some bulge-dominated spirals. Thus, our results suggest that the formation histories of galaxy bulges of early-type spirals can be significantly different from one galaxy to another.
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    Toward an understanding of the globular cluster overabundance around the central giant elliptical galaxy NGC 1399
    (1999) Kissler-Patig, M; Grillmair, CJ; Meylan, G; Brodie, JP; Minniti, D; Goudfrooij, P
    We investigate the kinematics of a combined sample of 74 globular clusters around NGC 1399. Their high velocity dispersion, increasing with radius, indicates their association with the gravitational potential of the galaxy cluster rather than with that of NGC 1399 itself. We find no evidence for rotation in the full sample, although there is some indication of rotation in the outer regions. The data do not allow us to detect differences between the kinematics of the blue and red subpopulations of globular clusters. A comparison of the globular cluster systems of NGC 1399 and those of NGC 1404 and NGC 1380 indicates that the globular clusters in all three galaxies are likely to have formed via similar mechanisms and at similar epochs. The only property that distinguishes the NGC 1399 globular cluster system from these others is that it is 10 times more abundant. We summarize the evidence for associating these excess globular clusters with the galaxy cluster rather than with NGC 1399 itself and suggest that the overabundance can be explained by tidal stripping, at an early epoch, of neighboring galaxies and subsequent accumulation of globular clusters in the gravitational potential of the galaxy cluster.