Searching for and characterizing halo substructures with the GALAH DR4 survey

TL;DR

This study uses GALAH DR4 and Gaia data to identify and analyze halo substructures in the Milky Way, revealing their origins through chemical and kinematic signatures.

Contribution

It introduces a combined wavelet and machine learning approach to characterize halo substructures and determine their extragalactic origins.

Findings

Recovered five halo structures, including GSE and Thamnos, with distinct chemo-dynamical properties.

Identified multiple peaks in GSE and Thamnos, indicating complex accretion histories.

Halo groups show signatures of extragalactic origins based on chemical and kinematic analysis.

Abstract

Recent studies show that the Milky Way stellar halo is composed of populations of different origins, shaped by multiple accretion events. To better understand the formation of the Milky Way and other spiral galaxies, we characterize the chemical and kinematic properties of halo substructures using GALAH DR4 and Gaia data. We apply wavelet transforms in the space of sqrt(J_r) and azimuthal action (L_z) to identify kinematic overdensities. Stars in the detected structures are analyzed in elemental abundance space to determine their origin. We further assess contamination using the unsupervised machine-learning algorithm t-distributed stochastic neighbor embedding (t-SNE), performing chemical tagging with 15 elemental abundances. We recover five structures: the Galactic disk, the Splash, Gaia-Sausage-Enceladus (GSE), Thamnos1, and Thamnos2. GSE shows two peaks; one at sqrt(J_r) ~ 25 kpc km s^-1 is due to disk contamination, while the other above sqrt(J_r) ~ 40 kpc km s^-1 represents the cleanest GSE population. Thamnos exhibits three peaks linked to Thamnos1 and Thamnos2. Thamnos2 shows higher [alpha/Fe], iron-peak elements are enhanced in the Splash, and halo groups retain a stronger r-process signature. The multiply peaked structures suggest that the splashed disk extends beyond prograde orbits. The distinct chemo-dynamical properties of the halo groups support their extragalactic origin.