The merger that led to the formation of the Milky Way's inner stellar halo and thick disk
Amina Helmi, Carine Babusiaux, Helmer H. Koppelman, Davide Massari,, Jovan Veljanoski, Anthony G.A. Brown

TL;DR
This study reveals that the Milky Way's inner halo and thick disk were primarily formed through a major merger with a galaxy called Gaia-Enceladus, which significantly influenced the Galaxy's structure about 10 billion years ago.
Contribution
It provides detailed chemo-dynamical evidence that a single major merger event shaped the inner halo and thick disk, identifying Gaia-Enceladus as the main progenitor.
Findings
Inner halo dominated by Gaia-Enceladus debris
Evidence of streams and retrograde motions in stellar orbits
Thick disk formed via dynamical heating from the merger
Abstract
The assembly process of our Galaxy can be retrieved using the motions and chemistry of individual stars. Chemo-dynamical studies of the nearby halo have long hinted at the presence of multiple components such as streams, clumps, duality and correlations between the stars' chemical abundances and orbital parameters. More recently, the analysis of two large stellar surveys have revealed the presence of a well-populated chemical elemental abundance sequence, of two distinct sequences in the colour-magnitude diagram, and of a prominent slightly retrograde kinematic structure all in the nearby halo, which may trace an important accretion event experienced by the Galaxy. Here report an analysis of the kinematics, chemistry, age and spatial distribution of stars in a relatively large volume around the Sun that are mainly linked to two major Galactic components, the thick disk and the stellar…
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