Contribution of Gaia Sausage to the Galactic Stellar Halo Revealed by K Giants and Blue Horizontal Branch Stars from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope, Sloan Digital Sky Survey, and Gaia

TL;DR

This study quantifies the Gaia-Sausage's significant contribution to the Milky Way's inner stellar halo using a Gaussian Mixture model applied to multiple stellar samples, revealing its decreasing influence with Galactocentric radius.

Contribution

It introduces a Gaussian Mixture model to distinguish the Gaia-Sausage component in the stellar halo and quantifies its contribution across different radii, providing new insights into halo composition.

Findings

Gaia-Sausage contributes 41%-74% to the inner halo within 30 kpc.

The contribution declines beyond 25-30 kpc, affecting the outer halo.

The GMM component aligns with substructure stars, confirming its reliability.

Abstract

We explore the contribution of the 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 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 Gaia-Sausage, and we find that the mean absolute radial velocity of the two lobes decreases with Galactocentric radius. We find that the Gaia-Sausage contributes about $41\%-74\%$ of the inner (Galactocentric radius $r_\mathrm{gc} < 30$ kpc) stellar halo. The fraction of stars of the 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 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.