Deciphering the Kinematic Substructure of Local Dark Matter with LAMOST K Giants

TL;DR

This study uses K giant stars from LAMOST and Gaia data to analyze the local dark matter velocity distribution, revealing significant deviations from the Standard Halo Model and implications for direct detection experiments.

Contribution

It introduces a chemodynamical approach to link stellar substructures with dark matter, providing a refined local DM velocity distribution model.

Findings

The GES-like substructure accounts for about 85% of local non-disk stars.

Approximately 25% of local dark matter is in kinematic substructure.

The modified velocity distribution peaks at lower speeds than the SHM, affecting detection limits.

Abstract

Numerical simulations indicate that correlations exist between the velocity distributions of stars and dark matter (DM). We study the local DM velocity distribution based on these correlations. We select K giants from LAMOST DR8 cross-matched with {\gaia} DR3, which have robust measurements of velocity and metallicity, and separate them into the disk, halo substructure and isotropic halo components in the chemodynamical space utilizing the Gaussian Mixture Model. The substructure component is highly radially anisotropic, and possibly related to the \gaia-Enceladus-Sausage (GES) merger event, while the isotropic halo component is accreted from the earliest mergers following the Maxwell-Boltzmann distribution (Standard Halo Model, SHM). We find that the GES-like substructure contributes $\sim85\%$ of the local nondisk stars in the Solar neighbourhood, which is nearly invariant when applying different volume cuts or additional angular momentum constraints. Utilizing the metallicity-stellar mass relation and the stellar mass-halo mass relation, we find that $\sim25_{-15}^{+24}\%$ of local DM is in the kinematic substructure. Combined with the stellar distributions of nondisk components, we modify the heliocentric velocity distribution of local DM. It shifts to a lower speed with a sharper peak compared to the SHM, and updates the detection limits of DM direct detection experiments. We discuss extensively the degeneracies present in the GMM fitting and propose that more kinematic and chemical information such as $\alpha$ abundance could help to break the degeneracy in the future. Our work confirms that the local DM velocity distribution deviates significantly from the SHM, and needs to be properly accounted for in the DM detection experiments.