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Identifying the Multiple Radial Mergers in the Local Stellar Halo with Chemodynamics

Presentation #203.03 in the session Mapping and Modeling the Milky Way’s Tidal Streams.

Published onApr 25, 2022
Identifying the Multiple Radial Mergers in the Local Stellar Halo with Chemodynamics

One of the seminal results from the Gaia survey was the discovery that most of the halo stars in the solar neighborhood were on radial orbits. It was conjectured that a large fraction of the halo was composed of one merger event that happened very early in the evolutionary history of the Milky Way, now known as the Gaia-Sausage/Enceladus merger. More recent results have questioned the number of merger remnants that contribute to the radial component of the halo, and also the timing of these infall events, though this remains unresolved. Different papers have assigned different names and used various selection criteria to identify the proposed radial merger events, including different criteria for the Gaia-Sausage/Enceladus merger. We found that dwarf stars within 2 kpc of the Sun separated into at least three components that were distinct in kinematics and photometrically determined metallicity. More recently, we used the dynamics and chemical abundances for APOGEE and GALAH stars within 5 kpc of the Sun to identify the same three distinct components. In the local solar region, the velocity and chemical structures originally attributed to the Gaia-Sausage/Enceladus merger are actually a combination of these three radial components. These mergers include a metal-rich but alpha-poor major merger that is consistent with a recent time of accretion (the Virgo Radial Merger), a metal-rich, alpha-rich, low-energy merger that is consistent with being accreted early on in the Galaxy’s history (Cronus), and a metal-poor, alpha-rich component of the halo that is consistent with either a recent major merger or many smaller radial mergers (Nereus).

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