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Two stellar components in the halo of the Milky Way

Author

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  • Daniela Carollo

    (INAF–Osservatorio Astronomico di Torino
    Center for the Study of Cosmic Evolution
    Joint Institute for Nuclear Astrophysics, Michigan State University, E. Lansing, Michigan 48824, USA
    Research School of Astronomy and Astrophysics, The Australian National University, Mount Stromlo Observatory, Cotter Road, Weston, Australian Capital Territory 2611, Australia)

  • Timothy C. Beers

    (Center for the Study of Cosmic Evolution
    Joint Institute for Nuclear Astrophysics, Michigan State University, E. Lansing, Michigan 48824, USA)

  • Young Sun Lee

    (Center for the Study of Cosmic Evolution
    Joint Institute for Nuclear Astrophysics, Michigan State University, E. Lansing, Michigan 48824, USA)

  • Masashi Chiba

    (Astronomical Institute, Tohoku University)

  • John E. Norris

    (Research School of Astronomy and Astrophysics, The Australian National University, Mount Stromlo Observatory, Cotter Road, Weston, Australian Capital Territory 2611, Australia)

  • Ronald Wilhelm

    (Texas Tech University, Lubbock, Texas 79409, USA)

  • Thirupathi Sivarani

    (Center for the Study of Cosmic Evolution
    Joint Institute for Nuclear Astrophysics, Michigan State University, E. Lansing, Michigan 48824, USA)

  • Brian Marsteller

    (Center for the Study of Cosmic Evolution
    Joint Institute for Nuclear Astrophysics, Michigan State University, E. Lansing, Michigan 48824, USA)

  • Jeffrey A. Munn

    (US Naval Observatory, PO Box 1149, Flagstaff, Arizona 86002, USA)

  • Coryn A. L. Bailer-Jones

    (Max-Planck-Institut für Astronomy, Königstuhl 17, D-69117, Heidelberg, Germany)

  • Paola Re Fiorentin

    (Max-Planck-Institut für Astronomy, Königstuhl 17, D-69117, Heidelberg, Germany
    University of Ljubljana, Jadranska 19, 1000, Ljubljana, Slovenia)

  • Donald G. York

    (Department of Astronomy and Astrophysics,
    The Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA)

Abstract

The halo of the Milky Way provides unique elemental abundance and kinematic information on the first objects to form in the Universe, and this information can be used to tightly constrain models of galaxy formation and evolution. Although the halo was once considered a single component, evidence for its dichotomy has slowly emerged in recent years from inspection of small samples of halo objects. Here we show that the halo is indeed clearly divisible into two broadly overlapping structural components—an inner and an outer halo—that exhibit different spatial density profiles, stellar orbits and stellar metallicities (abundances of elements heavier than helium). The inner halo has a modest net prograde rotation, whereas the outer halo exhibits a net retrograde rotation and a peak metallicity one-third that of the inner halo. These properties indicate that the individual halo components probably formed in fundamentally different ways, through successive dissipational (inner) and dissipationless (outer) mergers and tidal disruption of proto-Galactic clumps.

Suggested Citation

  • Daniela Carollo & Timothy C. Beers & Young Sun Lee & Masashi Chiba & John E. Norris & Ronald Wilhelm & Thirupathi Sivarani & Brian Marsteller & Jeffrey A. Munn & Coryn A. L. Bailer-Jones & Paola Re Fi, 2007. "Two stellar components in the halo of the Milky Way," Nature, Nature, vol. 450(7172), pages 1020-1025, December.
    • Handle: RePEc:nat:nature:v:450:y:2007:i:7172:d:10.1038_nature06460
    DOI: 10.1038/nature06460
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