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