Discovery of the shell structure via break radii in the outer halo of the Milky Way

TL;DR

This study uses Gaia DR3 RR Lyrae data to identify shell structures in the Milky Way's outer halo, revealing break radii linked to merger events and over-density shells indicative of past high-eccentricity mergers.

Contribution

It introduces improved methods to fit density profiles revealing shell structures and break radii associated with merger remnants in the Milky Way's outer halo.

Findings

Identification of two break radii coinciding with GSE merger apocenters.

Discovery of shell-shaped, metal-rich over-density structures beyond 50 kpc.

Ellipsoid axial ratio variation indicating complex halo substructures.

Abstract

Based on the \textit{Gaia} DR3 RR Lyrae catalog, we use two methods to fit the density profiles with an improved broken power law, and find that there are two break radii coinciding with the two apocenter pile-ups of high-eccentricity Gaia-Sausage-Enceladus (GSE) merger. Also, there is a break caused by the Sagittarius (Sgr) stream. Combining the positions of all breaks, we briefly analyze the metallicity and its dispersion as a function of $r$ as well as its distribution in cylindrical coordinates. For the clean sample, the $z\text{-to-}x$ ellipsoid axial ratio $q$ in $36\,{\rm kpc}\,\textless\,r\,\textless\,96\,{\rm kpc}$ becomes much smaller than that of the inner halo $(r\,\textless\,36\,{\rm kpc})$, while the major axis has a large uncertainty in the region of $36-66\,{\rm kpc}$ and the one in the region of $66-96\,{\rm kpc}$ is obviously different from that dominated by the Hercules-Aquila Cloud (HAC) and the Virgo Overdensity (VOD) in the inner halo, which indicates that there is an over-density structure distributed at low zenithal angles. Finally, we found that the over-density structure in the outer halo ($r\,\textgreater\,50\,{\rm kpc}$) is shell-shaped and relatively metal-rich compared to the outer background halo. We conclude that the shells could be the apocenter pile-ups of the high-eccentricity GSE merger, which is supported by previous numerical simulations.