title: Two stellar components in the halo of the Milky Way.
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.
authors: INAF-Osservatorio Astronomico di Torino, 10025 Pino Torinese, Italy. carollo@mso.anu.edu.au
authors: Daniela
authors: D
authors: Carollo
authors: Timothy C
authors: TC
authors: Beers
authors: Young Sun
authors: YS
authors: Lee
authors: Masashi
authors: M
authors: Chiba
authors: John E
authors: JE
authors: Norris
authors: Ronald
authors: R
authors: Wilhelm
authors: Thirupathi
authors: T
authors: Sivarani
authors: Brian
authors: B
authors: Marsteller
authors: Jeffrey A
authors: JA
authors: Munn
authors: Coryn A L
authors: CA
authors: Bailer-Jones
authors: Paola Re
authors: PR
authors: Fiorentin
authors: Donald G
authors: DG
authors: York