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Two stellar components in the halo of the Milky Way

An Erratum to this article was published on 10 January 2008


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.

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Figure 1: The spatial distribution of the stars analysed in the present sample.
Figure 2: The distribution of [Fe/H] for various cuts in the V velocity (in km s -1 ), the component of orbital motion measured with respect to the Local Standard of Rest.
Figure 3: The distribution of [Fe/H] for the stars in our sample on highly retrograde orbits.
Figure 4: A sample of blue horizontal-branch stars exploring much larger distances from the Galactic Centre than the SDSS calibration stars.
Figure 5: Equidensity contours of the reconstructed global density distributions for stars in our sample with various metallicities.


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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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Correspondence to Daniela Carollo.

Supplementary information

Supplementary Information

This file presents details concerning the selection of the stars used in this analysis, as well as for the derivation of the stellar atmospheric parameters, and for the derivation of the kinematic parameters. Additional kinematic analyses are also reported. The file contains Supplementary Notes, Supplementary Tables 1-2 and Supplementary Figures 1-6 with Legends. (PDF 1206 kb)

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Carollo, D., Beers, T., Lee, Y. et al. Two stellar components in the halo of the Milky Way. Nature 450, 1020–1025 (2007).

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