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, Donald G. York

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    490 Citations (Scopus)


    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.

    Original languageEnglish
    Pages (from-to)1020-1025
    Number of pages6
    Issue number7172
    Publication statusPublished - 13 Dec 2007


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