Two stellar components in the halo of the Milky Way

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 Fiorentin and Donald G. York
Additional contact information
Daniela Carollo: INAF–Osservatorio Astronomico di Torino
Timothy C. Beers: Center for the Study of Cosmic Evolution
Young Sun Lee: Center for the Study of Cosmic Evolution
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
Brian Marsteller: Center for the Study of Cosmic Evolution
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
Donald G. York: Department of Astronomy and Astrophysics,

Nature, 2007, vol. 450, issue 7172, 1020-1025

Abstract: 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.

Date: 2007
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DOI: 10.1038/nature06460

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