Detection of Multiple Phase Space Overdensities of GSE Stars by Orbit Integration

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 10(12) M (circle dot). 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.