Editor's Summary

13 December 2007

Halo, halo

The outer region of the Milky Way beyond the galactic disk, known as the halo, was long thought of as a homogenous entity, made up of ancient stars. But recent analysis of small numbers of objects within the halo suggests that they do not comprise a single population. Based on spectroscopic data from more than 20,000 stars, the halo is shown to consist of two broadly overlapping structural components — an inner halo that rotates slowly in the same direction as the Milky Way as a whole; and an outer halo rotating in the opposite direction. The outer halo has relatively low abundances of elements heavier than helium. The inner halo may have formed by a succession of dissipational mergers, and the outer halo through dissipationless processes and the tidal disruption of proto-galactic clumps.

Article: 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

doi:10.1038/nature06460

Abstract | Full Text | PDF (497K) | Supplementary information