Galactic Disk Bulk Motions as Revealed by the LSS-GAC DR2
  We report a detailed investigation of the bulk motions of the nearby Galactic
stellar disk, based on three samples selected from the LSS-GAC DR2: a global
sample containing 0.57 million FGK dwarfs out to $\sim$ 2 kpc, a local subset
of the global sample consisting $\sim$ 5,400 stars within 150 pc, and an
anti-center sample containing $\sim$ 4,400 AFGK dwarfs and red clump stars
within windows of a few degree wide centered on the Galactic anti-center. The
global sample is used to construct a three-dimensional map of bulk motions of
the Galactic disk from the solar vicinity out to $\sim$ 2 kpc with a spatial
resolution of $\sim$ 250 pc. Typical values of the radial and vertical
components of bulk motion range from $-$15 km s$^{-1}$ to 15 km s$^{-1}$, while
the lag behind the circular speed dominates the azimuthal component by up to
$\sim$ 15 km s$^{-1}$. The map reveals spatially coherent, kpc-scale stellar
flows in the disk, with typical velocities of a few tens km s$^{-1}$. Bending-
and breathing-mode perturbations are clearly visible, and vary smoothly across
the disk plane. Our data also reveal higher-order perturbations, such as breaks
and ripples, in the profiles of vertical motion versus height. From the local
sample, we find that stars of different populations exhibit very different
patterns of bulk motion. Finally, the anti-center sample reveals a number of
peaks in stellar number density in the line-of-sight velocity versus distance
distribution, with the nearer ones apparently related to the known moving
groups. The "velocity bifurcation" reported by Liu et al. (2012) at
Galactocentric radii 10--11 kpc is confirmed. However, just beyond this
distance, our data also reveal a new triple-peaked structure.
