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

Context. 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. With the advent of Gaia Data Release 3 (DR3), which includes high-precision positions, parallaxes, proper motions, radial velocities, the identification and characterisation of thousands of RRLs, it has become possible to study the distribution, kinematics, and metallicity of RRLs in the various dynamical substructures with unprecedented detail. Aims. Our primary goal is to identify and characterise the dynamical substructures of the MW halo using RRLs from Gaia DR3. Methods. 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 then used newly calibrated photometric metallicities available in the literature to study the metallicity distributions of RRLs in different substructures. Results. 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] = −1.64 ± 0.26 dex), and the Helmi streams ([Fe/H] = −1.66 ± 0.19 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±0.20 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 overdensities 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.