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