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 journalArticle

412 Citations (Scopus)

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.

Original languageEnglish
Pages (from-to)1020-1025
Number of pages6
JournalNature
Volume450
Issue number7172
DOIs
Publication statusPublished - 13 Dec 2007

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    Carollo, D., Beers, T. C., Lee, Y. S., Chiba, M., Norris, J. E., Wilhelm, R., ... York, D. G. (2007). Two stellar components in the halo of the Milky Way. Nature, 450(7172), 1020-1025. https://doi.org/10.1038/nature06460