Two stellar components in the halo of the Milky Way

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. 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. The halo of the Milky Way is clearly divisible into two broadly overlapping structural components, an inner and an outer halo. While the inner halo has a modest net prograde rotation, the outer halo exhibits a net retrograde rotation and a peak metallicity one third that of the inner.