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

TL;DR

This study uses 3,003 RR Lyrae stars with 6D data to identify and analyze substructures in the Milky Way's halo, revealing known, new, and possibly associated substructures, and providing insights into their distribution and origins.

Contribution

The paper introduces a method to identify halo substructures using integrals of motion and applies it to a large RR Lyrae dataset, discovering new substructures and analyzing the properties of known ones.

Findings

Identified multiple known and new substructures in the Galactic halo.

Found that Gaia-Enceladus-Sausage dominates the inner halo and influences the density profile.

Suggested similar origins for GES, Hercules-Aquila Cloud, and Virgo Overdensity.

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 $\rm -2.13\,dex$ 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 $34\,\rm kpc$, which suggests that the GES is mainly distributed in the inner halo. The near one-third proportion of the GES and the peak value $20\,\rm kpc$ of the apocenter distances suggest that GES could account for the break in the density profile of the Galactic halo at Galactocentric distance ${\sim}20-25\,\rm kpc$. 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.