Examinando por Autor "Harris, William E."

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  • Miniatura
    Ítem
    Globular cluster scale sizes in giant galaxies: orbital anisotropy and tidally underfilling clusters in M87, NGC 1399 and NGC 5128
    (OXFORD UNIV PRESS, 2016-08) Webb, Jeremy J.; Sills, Alison; Harris, William E.; Gómez, Matías; Paolillo, Maurizio; Woodley, Kristin A.; Puzia, Thomas H.
    We investigate the shallow increase in globular cluster half-light radii with projected galactocentric distance Rgc observed in the giant galaxies M87, NGC 1399, and NGC 5128. To model the trend in each galaxy, we explore the effects of orbital anisotropy and tidally underfilling clusters. While a strong degeneracy exists between the two parameters, we use kinematic studies to help constrain the distance Rβ beyond which cluster orbits become anisotropic, as well as the distance Rfα beyond which clusters are tidally underfilling. For M87 we find Rβ > 27 kpc and 20 < Rfα < 40 kpc and for NGC 1399 Rβ > 13 kpc and 10 < Rfα < 30 kpc. The connection of Rfα with each galaxy's mass profile indicates the relationship between size and Rgc may be imposed at formation, with only inner clusters being tidally affected. The best-fitting models suggest the dynamical histories of brightest cluster galaxies yield similar present-day distributions of cluster properties. For NGC 5128, the central giant in a small galaxy group, we find Rβ > 5 kpc and Rfα > 30 kpc. While we cannot rule out a dependence on Rgc, NGC 5128 is well fitted by a tidally filling cluster population with an isotropic distribution of orbits, suggesting it may have formed via an initial fast accretion phase. Perturbations from the surrounding environment may also affect a galaxy's orbital anisotropy profile, as outer clusters in M87 and NGC 1399 have primarily radial orbits while outer NGC 5128 clusters remain isotropic.
  • Miniatura
    Ítem
    Structural Parameters of the M87 Globular Clusters
    (2009) Madrid, Juan P.; Harris, William E.; Blakeslee, John P.; Gómez, Matías
    We derive structural parameters for ∼ 2000 globular clusters in the giant Virgo elliptical M87 using extremely deep Hubble Space Telescope images in F606W (V) and F814W (I) taken with the ACS/WFC. The cluster scale sizes (half-light radii rh) and ellipticities are determined from PSF-convolved King-model profile fitting. We find that the rh distribution closely resembles the inner Milky Way clusters, peaking at rh ≃ 2.5 pc and with virtually no clusters more compact than rh ≃ 1 pc. The metal-poor clusters have on average an rh 24% larger than the metal-rich ones. The cluster scale size shows a gradual and noticeable increase with galactocentric distance. Clusters are very slightly larger in the bluer waveband V , a possible hint that we may be beginning to see the effects of mass segregation within the clusters. We also derived a color magnitude diagram for the M87 globular cluster system which show a striking bimodal distribution.