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

Abstract

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. 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. Using t-distributed stochastic neighbour embedding (t-SNE), we performed dimensionality reduction and identify stars from clusters and streams that are co-localised in the kinematic and chemical parameter space. We included E, Jr, Jz, Lz, r_apo, r_peri, and eccentricity as well as [Fe/H] as input into the algorithm, and used a sample of 5347 stars from 229 individual Milky Way substructures compiled from various sources in the literature. 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 another work. We find that 44\% of Milky Way globular clusters are consistent with having an accreted origin. We also find that the chemo-dynamic properties of omega cen are consistent with a common accretion with the Thamnos structure. In addition, we identified many stream-progenitor associations, most notably 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.