The Local Stellar Halo is Not Dominated by a Single Radial Merger Event

TL;DR

This study analyzes local halo dwarf stars near the Sun to reveal multiple substructures and merger remnants, challenging the idea of a single dominant radial merger event shaping the stellar halo.

Contribution

It identifies and characterizes multiple halo substructures and merger remnants, providing a detailed view of the local stellar halo's complex assembly history.

Findings

Multiple RME components with distinct metallicities and energies identified.

The Gaia Sausage is a combination of several substructures.

Presence of metal-poor halo stars and a disk/Splash component.

Abstract

We use halo dwarf stars with photometrically determined metallicities that are located within 2 kpc of the Sun to identify local halo substructure. The kinematic properties of these stars do not indicate a single, dominant radial merger event (RME). The retrograde Virgo Radial Merger (VRM) component has [Fe/H] = -1.7. A second, non-rotating RME component we name Nereus is identified with [Fe/H] = -2.1 and has similar energy as the VRM. A possible third RME we name Cronus is identified that is co-rotating with the disk, has lower energy than the VRM, and has [Fe/H] = -1.2. We identify the Nyx Stream in the data. In addition to these substructures, we observe metal-poor halo stars ([Fe/H] ~ -2.0 and $\sigma_v$ ~ 180 km s$^{-1}$) and a disk/Splash component with lower rotational velocity than the disk and lower metallicity than typically associated with the Splash. An additional excess of halo stars with low velocity and metallicity of [Fe/H] = -1.5 could be associated with the shell of a lower energy RME or indicate that lower energy halo stars have higher metallicity. Stars which comprise the "Gaia Sausage" velocity structure are a combination of the components identified in this work.