Searching for and characterizing halo substructures with the GALAH DR4 survey

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.

Figures (9)

• Figure 1: Galactocentric distance, R, as a function of distance from the Galactic plane, Z, in the direction of the North Galactic Pole, Z $=90^{\circ}$, for 124 618 stars selected from GALAH DR4. Dashed lines show the Solar values R $=8$ kpc, and Z $=20.8$ pc and the yellow star shows the location of the Sun. The bin size is $0.05\times0.05$ kpc.
• Figure 2: Panel (a): Density map in $L_z - \sqrt{J_r}$ space of 124 618 stars that were selected from GALAH DR4. The bin size is $11.7\times0.2$ kpc ${\rm\,km\,s^{-1}}$. Panel (b): The wavelet transform map of 500 Monte-Carlo-generated samples in the $L_z - \sqrt{J_r}$ space for scale $J=5$. Centres of the detected structures are shown with white crosses. The structures are the disk, Splash, GSE, and Thamnos. Panel (c): The wavelet transform map of 500 Monte-Carlo-generated samples in the $L_z - \sqrt{J_r}$ space for scale $J=6$. Centres of the detected structures are shown with white crosses. GSE and the disk are the two main building blocks. Panel (d): The same as panel (b), but in a gray scale. The ellipses show areas in the $L_z - \sqrt{J_r}$ space where we select stars to further analyze the detected kinematic structures.
• Figure 3: The binned distributions of 116 047 stars selected from the GALAH DR4 in the [Mg/Cu] – [Na/Fe] plane. The bin size is 0.01 × 0.01. The dashed line shows the division of stars into the disk (below the line) and halo (above the line) stars.
• Figure 4: A scatter plot of all 116 047 stars selected from GALAH DR4 in the [Mg/Cu] -- [Na/Fe] plane is shown in gray. The markers of different colors show stars in the following kinematic structures: Splash (Panel (a), orange circles), GSE (Panel (b), blue circles), Thamnos1 (Panel (c), green circles), and Thamnos2 (Panel (d), red circles). The dashed line is the same as on panel (a) and divides the stars into the disk and halo sub-samples. Percentages of the halo and disk stars in the kinematic structures are provided in each plot.
• Figure 5: Panel (a): The plot shows the orbital energy as a function of the angular momentum. Gray dots show a scatter plot of the total sample, and other colors and symbols correspond to the kinematic structures detected in the legend. Chemically-defined halo stars were excluded from the Splash, and chemically-defined disk stars were excluded from the rest of the groups. Panel (b): Similar to the plot on panel (a), but showing the rotational velocity as a function of metallicity.