# Identifying Galactic Halo Substructure in 6D Phase-space Using $\sim$13,000 LAMOST K Giants

## TL;DR

This study combines LAMOST and Gaia data to detect and analyze substructures in the Milky Way's halo, revealing increased substructure with distance and metallicity and identifying known and new stellar streams.

## Contribution

It introduces a large 6D phase-space halo K-giant sample and applies clustering algorithms to identify and characterize halo substructures, including the first spectroscopic members of the Monoceros Ring.

## Key findings

- Significant position-velocity substructure detected in the stellar halo.
- Identification of known streams and discovery of new substructures.
- First spectroscopic detection of Monoceros Ring members with specific kinematic and metallicity properties.

## Abstract

We construct a large halo K-giant sample by combining the positions, distances, radial velocities, and metallicities of over 13,000 LAMOST DR5 halo K giants with the Gaia DR2 proper motions, which covers a Galactocentric distance range of 5-120 kpc. Using a position-velocity clustering estimator (the 6Distance), we statistically quantify the presence of position-velocity substructure at high significance: K giants have more close pairs in position-velocity space than a smooth stellar halo. We find that the amount of substructure in the halo increases with increasing distance and metallicity. With a percolation algorithm named friends-of-friends (FoF) to identify groups, we identify members belonging to Sagittarius (Sgr) Streams, Monoceros Ring, Virgo overdensity, Hercules-Aquila Cloud, Orphan Streams and other unknown substructures and find that the Sgr streams account for a large part of grouped stars beyond 20 kpc and enhance the increase of substructure with distance and metallicity. For the first time, we identify spectroscopic members of Monoceros Ring in the south and north Galactic hemisphere, which presents a rotation of about 185 km s^{-1} and mean metallicity is -0.66 dex.