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Debris streams in the solar neighbourhood as relicts from the formation of the Milky Way

Nature volume 402, pages 53–55 (1999)Cite this article

Abstract

It is now generally believed that galaxies were built up through gravitational amplification of primordial fluctuations and the subsequent merging of smaller precursor structures. The stars of the structures that assembled to form the Milky Way should make up much or all of its bulge and halo, in which case one hopes to find ‘fossil’ evidence for those precursor structures in the present distribution of halo stars. Confirmation that this process is continuing came with the discovery of the Saggittarius dwarf galaxy1, which is being disrupted by the Milky Way, but direct evidence that this process provided the bulk of the Milky Way's population of old stars has hitherto been lacking. Here we show that about ten per cent of the metal-poor stars in the halo of the Milky Way, outside the radius of the Sun's orbit, come from a single coherent structure that was disrupted during or soon after the Galaxy's formation. This object had a highly inclined orbit about the Milky Way at a maximum distance of ∼16 kpc, and it probably resembled the Fornax and Sagittarius dwarf spheroidal galaxies.

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Figure 1: The distribution of nearby halo stars in the plane of angular momentum components, Jz versus J⊥ = √(J2x + J2y).
Figure 2: The distribution of nearby halo stars in velocity space and in the Jz–J⊥ plane.

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Acknowledgements

A.H. thanks the Max-Planck-Institut für Astrophysik for hospitality during her visits. We made use of the Simbad database (maintained by Centre de Données astronomiques de Strasbourg) and of the HIPPARCOS online facility at the European Space Research and Technology Centre (ESTEC) of the European Space Agency (ESA).

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Authors and Affiliations

  1. Leiden Observatory, PO Box 9513, Leiden, 2300 RA, The Netherlands

    Amina Helmi, P. Tim de Zeeuw & HongSheng Zhao

  2. Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Strasse 1, Garching bei München, 85740, Germany

    Simon D. M. White

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  1. Amina Helmi
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Correspondence to Amina Helmi.

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Helmi, A., White, S., de Zeeuw, P. et al. Debris streams in the solar neighbourhood as relicts from the formation of the Milky Way. Nature 402, 53–55 (1999). https://doi.org/10.1038/46980

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