Influence of the Gaia-Sausage-Enceladus on the density shape of the Galactic stellar halo revealed by halo K giants from the LAMOST survey

TL;DR

This study investigates how the Gaia-Sausage-Enceladus event influences the shape of the Milky Way's stellar halo, revealing differences in flattening and density profiles between GSE-related and GSE-removed components using LAMOST K giants.

Contribution

It provides a novel analysis of the GSE's impact on halo shape by dividing the halo into GSE-related and GSE-removed parts and comparing their density profiles and flattening.

Findings

GSE-related halo is less flattened than GSE-removed halo.

Both halos become more spherical at larger radii.

Major merger analogues show similar halo shape differences.

Abstract

We present a study of the influence of the Gaia-Sausage-Enceladus (GSE) on the density shape of the Galactic stellar halo using 11624 K giants from the LAMOST survey. Every star is assigned a probability of being a member of the GSE based on its spherical velocities and metallicity by a Gaussian Mixture Model. We divide the stellar halo into two parts by the obtained probabilities, of which one is composed of the GSE members and defined as the GSE-related halo, and the other one is referred to as the GSE-removed halo. Using a non-parametric method, the radial number density profiles of the two stellar halos can be well described by a single power law with a variable flattening $q$ ($r = \sqrt{R^2+[(Z/q(r))]^2}, \nu = {\nu_0}r^{-\alpha}$). The index $\alpha$ is $4.92\pm0.12$ for the GSE-related halo and $4.25\pm0.14$ for the GSE-removed halo. Both the two stellar halos are vertically flattened at smaller radii but become more spherical at larger radii. We find that the GSE-related halo is less vertically flattened than the GSE-removed halo, and the difference of $q$ between the two stellar halos ranges from 0.07 to 0.15. However, after the consideration of the bias, it is thought to be within 0.08 at most of the radii. Finally, we compare our results with two Milky Way analogues which experience a significant major merger in the TNG50 simulation. The study of the two analogues also shows that the major merger-related stellar halo has a smaller ellipticity than the major merger-removed stellar halo.