Milky Way halo Gaia DR3 RR Lyrae stars

We applied the domain-informed novelty detection CLustering in Multiphase Boundaries (CLiMB) framework to identify RRL membership in the MW sub-structures. We then used newly calibrated photometric metallicities available in the literature to study the metallicity distributions of RRLs in different sub-structures. 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+/-0.25dex), Sequoia ([Fe/H]=-1.64+/-0.26dex), and the Helmi streams ([Fe/H]=-1.66+/-0.19dex). The metallicity distribution of RRLs in Thamnos was found to be bimodal, with the metal-poor peak likely representing the genuine (i.e. true) accreted Thamnos population ([Fe/H]=-1.94+/-0.20dex), in agreement with recent works based on spectroscopic abundances.

Our analysis shows that the sub-structures ED-1 and L-RL3 are highly contaminated by thick-disc stars. However, the metal-poor tails in their metallicity distributions might be signatures of remnants from small accreted systems. We also identified some over-densities of RRLs in correspondence with the recently reported sub-structures Shiva and Shakti, which we suggest are of an in situ origin.

Finally, we applied the RRL-based mass-metallicity relation of galaxies to test the nature of the identified dynamical sub-structures.