Probing the Galactic Halo with RR Lyrae Stars. II. The Substructures of the Milky Way

Abstract

We identify substructures of the Galactic halo using 3,003 type $ab$ RR Lyraes (RRab) with 6D position-velocity information from the SDSS, LAMOST, and Gaia EDR3. Based on the information, we define the separation of any two of the stars in the integrals of motion space and identify substructures by utilizing the friends-of-friends algorithm. We identify members belonging to several known substructures: the Sagittarius stream, the Gaia-Enceladus-Sausage (GES), the Sequoia, and the Helmi streams. In addition to these known substructures, there are three other substructures possibly associated with globular clusters NGC 5272, NGC 6656, and NGC 5024, respectively. Finally, we also find three remaining unknown substructures and one of them has large angular momentum and a mean metallicity $-2.13\mathrm{dex}$ which may be a new substructure. As for GES, we find that it accounts for a large part of substructures in the inner halo and the range of apocenter distance is from 10 to $34\mathrm{kpc}$, which suggests that the GES is mainly distributed in the inner halo. The near one-third proportion of the GES and the peak value $20\mathrm{kpc}$ of the apocenter distances suggest that GES could account for the break in the density profile of the Galactic halo at Galactocentric distance $\sim 20-25\mathrm{kpc}$. The similarity of comparing the kinematic properties of Gaia-Enceladus-Sausage with the Hercules-Aquila Cloud and Virgo Overdensity suggests that the three substructures may have similar origins.

14 pages, 14 figures, accepted by MNRAS

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• A high fidelity Milky Way simulation with Kraken, Gaia-Enceladus, and Sequoia analogues: clues to their accretion histories
• Hidden deep in the halo: selection of a reduced proper motion halo catalogue and mining retrograde streams in the velocity space
• A determination of the LMC dark matter subhalo mass using the MW halo stars in its gravitational wake
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• The Chemodynamical Nature of the Triangulum-Andromeda Overdensity
• A Vertically Orientated Dark Matter Halo Marks a Flip of the Galactic Disk
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• Reconsidering The Bailey Diagrams of ab-type RR Lyrae Stars, Metallicity-Mediated Evolution as the Direct Cause of the Oosterhoff Phenomenon
• Unveiling Galactic substructures with M Giant stars: A kinematic and chemical study based on LAMOST DR9, Gaia DR3 and APOGEE DR17
• Uncover 3D Dark Matter Distribution of the Milky Way by an Empirical Triaxial Orbit-Superposition Model: Method Validation
• RR Lyrae stars with variable mean magnitudes