1. INAF–Osservatorio Astronomico di Torino, 10025 Pino Torinese, Italy
2. Department of Physics and Astronomy, Center for the Study of Cosmic Evolution,
3. Joint Institute for Nuclear Astrophysics, Michigan State University, E. Lansing, Michigan 48824, USA
4. Astronomical Institute, Tohoku University, Sendai 980-8578, Japan
5. Research School of Astronomy and Astrophysics, The Australian National University, Mount Stromlo Observatory, Cotter Road, Weston, Australian Capital Territory 2611, Australia
6. Department of Physics, Texas Tech University, Lubbock, Texas 79409, USA
7. US Naval Observatory, PO Box 1149, Flagstaff, Arizona 86002, USA
8. Max-Planck-Institut für Astronomy, Königstuhl 17, D-69117, Heidelberg, Germany
9. Department of Physics, University of Ljubljana, Jadranska 19, 1000, Ljubljana, Slovenia
10. Department of Astronomy and Astrophysics,
11. The Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA

Correspondence to: Daniela Carollo^1,2,3,5 Correspondence and requests for materials should be addressed to D.C. (Email: carollo@mso.anu.edu.au).

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.

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