Contribution of Gaia-Sausage to the Galactic stellar halo revealed by K giants and BHB stars from LAMOST, SDSS, and Gaia

We explore the contribution of the \textit{Gaia}-Sausage to the stellar halo of the Milky Way by making use of a Gaussian Mixture model (GMM) and applying it to halo star samples of LAMOST K giants, SEGUE K giants, and SDSS blue horizontal branch stars. The GMM divides the stellar halo into two parts, of which one represents a more metal-rich and highly radially biased component associated with an ancient, head-on collision referred to as the \textit{Gaia}-Sausage, and the other one is a more metal-poor and isotropic halo. A symmetric bimodal Gaussian is used to describe the distribution of spherical velocity of the \textit{Gaia}-Sausage, and we find that the mean absolute radial velocity of the two lobes decreases with Galactocentric radius. We find that the \textit{Gaia}-Sausage contributes about $41\%-74\%$ of the inner (Galactocentric radius $r_\mathrm{gc} < 30$ kpc) stellar halo. The fraction of stars of the \textit{Gaia}-Sausage starts to decline beyond $r_\mathrm{gc} \sim$ $25-30$ kpc, and the outer halo is found to be significantly less influenced by the \textit{Gaia}-Sausage than the inner halo. After the removal of halo substructures found by integrals of motion, the contribution of the \textit{Gaia}-Sausage falls slightly within $r_\mathrm{gc} \sim$ 25 kpc, but is still as high as $30\%-63\%$. Finally, we select several possible Sausage-related substructures consisting of stars on highly eccentric orbits. The GMM/Sausage component agrees well with the selected substructure stars in their chemodynamical properties, which increases our confidence in the reliability of the GMM fits.