Two Stellar Components in the Halo of the Milky Way

TL;DR

The Milky Way's halo consists of two distinct components, an inner and an outer halo, with different structures, rotations, and metallicities, revealing different formation histories.

Contribution

This study provides clear evidence for the dichotomy of the Milky Way halo into two components with distinct properties and formation processes.

Findings

Inner halo has a modest prograde rotation.

Outer halo exhibits a net retrograde rotation.

Outer halo has significantly lower metallicity than the inner halo.

Abstract

The halo of the Milky Way provides unique elemental abundance and kinematic information on the first objects to form in the Universe, which 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.