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

Research output: Contribution to journalArticlepeer-review

500 Scopus citations


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
StatePublished - Dec 13 2007

Bibliographical note

Funding Information:
Acknowledgements We thank C. Allende Preito, E. Bell, W. Brown, A. Frebel, B. Gibson, H. Morrison, C. Thom, J. Tumlinson and B. Yanny for comments on previous versions of this Article. D.C. acknowledges partial support for travel and living expenses from JINA, the Joint Institute for Nuclear Astrophysics, while in residence at Michigan State University. Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the US Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England. The SDSS website is

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