Galactic disk bulk motions as revealed by the LSS-GAC DR2

Abstract

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 similar to 2 kpc, a local subset of the global sample consisting of similar to 5400 stars within 150 pc, and an anti-center sample containing similar to 4400 AFGK dwarfs and red clump stars within windows a few degrees 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 similar to 2 kpc with a spatial resolution of similar to 250 pc. Typical values of the radial and vertical components of bulk motion range from -15 km s(-1) to 15 km s(-1); in contrast, the lag behind the circular motion dominates the azimuthal component by up to similar to 15 km s(-1). The map reveals spatially coherent, kpc-scale stellar flows in the disk, with typical velocities of a few tens of 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 from 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. at Galactocentric radii 10-11 kpc is confirmed. However, just beyond this distance, our data also reveal a new triple-peaked structure.