Probing the Milky Way Halo with RR Lyrae Stars from Gaia Data Release 3

Abstract

The Milky Way (MW) stellar halo, containing debris from past accretion events, serves as a fossil record of hierarchical mass assembly. Due to their distinct properties, RR Lyrae stars (RRLs) serve as excellent tracers for identifying and characterising the halo's substructures. We analysed a sample of 4933 RRLs, for which we calculated the integrals of motion and orbital parameters. We applied the domain-informed novelty detection CLustering in Multiphase Boundaries (CLiMB) framework to identify RRL membership in the MW substructures. We analysed the metallicity distributions of RRLs in major accreted system remnants as a snapshot of their chemical evolutionary status during early epochs. We calculated the weighted mean metallicity ([Fe/H]) and the corresponding standard deviation for Gaia Sausage/Enceladus ([Fe/H] = $-1.57 \pm 0.25$ dex), Sequoia ([Fe/H] =$ -1.64\pm0.26$ dex), and the Helmi streams ([Fe/H] = $-1.66\pm0.19$ dex). The metallicity distribution of RRLs in Thamnos was found to be bimodal, with the metal-poor peak likely representing the genuine accreted Thamnos population ([Fe/H] = $-1.94\pm0.20$ dex), in agreement with recent works based on spectroscopic abundances. Our analysis shows that the substructures ED-1 and L-RL3 are highly contaminated by thick disc stars. However, the metal-poor tails in their metallicity distributions may be signatures of remnants from small accreted systems. We also identify over-densities of RRLs in correspondence with the recently reported substructures Shiva and Shakti, which we suggest are of in-situ origin. Finally, we applied the RRL-based mass-metallicity relation of galaxies to test the nature of the identified dynamical substructures.