2023ApJ...950..151W

In N-body simulations, nearly radial mergers can form shell-like overdensities in the sky position and phase space (r - v_r) due to the combination of dynamical friction and tidal stripping. The merger event of Gaia-Sausage-Enceladus (GSE) has provided a unique opportunity to study the shells in the phase space. To search for them, we integrate the orbits of 5949 GSE-related halo K giants from the LAMOST survey and record their positions at all time intervals in the r - v_r diagram. After the subtraction of a smoothed background, we find six significant and complete thin chevron-like overdensities. The apocenters r_apo of stars in the six chevrons are around 6.75, 12.75, 18.75, 25.25, 27.25, and 30.25 kpc. These chevrons reveal the multiple pileups of GSE stars at different apocenters. The application of a different Milky Way mass M_vir will change the opening angles of these chevrons, while leaving their apocenters almost unchanged. By comparing with a recent study of the phase space overdensities of local halo stars from the Gaia Radial Velocity Spectrometer survey, our results are more inclined to a medium M_vir of 10¹²M_⊙. The application of a nonaxisymmetric Galactic potential with a steadily rotating bar has a blurring effect on the appearance of these chevron-like overdensities, especially for the chevrons with r_apo > 20 kpc.