2025ApJ...979..213S

Motivated by the vast gap between photometric and spectroscopic data volumes, there is great potential in using 5D kinematic information to identify and study substructures of the Milky Way. We identify substructures in the Galactic halo using 46,575 RR Lyrae stars (RRLs) from Gaia Data Release 3, with photometric metallicities and distances newly estimated by X.-Y. Li et al. Assuming a Gaussian prior distribution of radial velocity, we calculate the orbital distribution characterized by the integrals of motion for each RRL, based on its 3D positions, proper motions, and corresponding errors, and then apply the friends-of-friends algorithm to identify groups moving along similar orbits. We have identified several known substructures, including the Sagittarius stream, the Hercules-Aquila Cloud (HAC), the Virgo Overdensity (VOD), Gaia-Enceladus-Sausage (GES), the Orphan-Chenab stream, Cetus-Palca, the Helmi streams, Sequoia, Wukong, and the Large Magellanic Cloud leading arm, as well as 18 unknown groups. Our findings indicate that the HAC and VOD have kinematic and chemical properties remarkably similar to GES, with most HAC and VOD members exhibiting eccentricity as high as GES, suggesting that they may share a common origin with GES. The ability to identify low-mass and spatially dispersed substructures further demonstrates the potential of our method, which breaks the limit of spectroscopic surveys and is competent for probing substructures throughout the whole Galaxy. Finally, we also identify 18 unknown groups with good spatial clustering and proper-motion consistency, suggesting the possibility of further excavation of Milky Way substructures in the future with only 5D data.