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 0. 25 dex), Sequoia (Fe/H = -1. 640. 26 dex), and the Helmi streams (Fe/H = -1. 660. 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. 940. 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.
Muraveva et al. (Mon,) studied this question.