Identifying substructure associations in the Milky Way halo using chemo-kinematic tagging

Department of Astronomy, Stockholm University, AlbaNova University Centre, Roslagstullsbacken 21, 106 91 Stockholm, Sweden

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 1 April 2025
Accepted: 17 September 2025

Abstract

Context. The Milky Way halo has been built-up over cosmic time through the accretion and dissolution of star clusters and dwarf galaxies as well as through their complex interactions with the Galactic disc. Traces of these accreted structures persist to the present day in the chemical and kinematic properties of stars and their orbits and allow for the disentangling of the accretion history of the Galaxy through observations of Milky Way stars.

Aims. We utilised 6D phase-space information in combination with [Fe/H] measurements to facilitate a clustering analysis of stars using their kinematics and chemistry simultaneously, a technique known as chemo-kinematic tagging. We aim to associate large halo substructure groups with Milky Way halo globular clusters, stellar streams, and satellite galaxies in order to investigate the common origins of these groups of structures.

Methods. We implemented t-distributed stochastic neighbour embedding (t-SNE) to perform dimensionality reduction and identify stars from clusters and streams that are co-localised in the kinematic and chemical parameter space. We used the orbital parameters E, J_r, J_z, L_z, r_apo, r_peri, and eccentricity as well as [Fe/H] as input into the algorithm, and we performed a clustering analysis for a sample of 5347 stars from 229 individual Milky Way substructures.

Results. Most notably, we recovered several large-scale structures that have been reported in the literature, including GSE, Thamnos, Sequoia, I’itoi, LMS-1/Wukong, Sagittarius, Kraken/Koala, the splashed disc, and a candidate structure recently found in the literature. We assigned globular cluster populations to each of these accreted structures and find that 44% of Milky Way globular clusters are consistent with having an accreted origin. In addition, we find that the chemo-dynamic properties of Omega Cen are consistent with a common accretion with the Thamnos structure. Finally, we identified many small-scale structures, including several stream-progenitor associations, and a connection between the Orphan-Chenab stream and the Grus II ultra-faint dwarf galaxy, which supports previous findings that these two objects were brought into the Galaxy in the same accretion event.