Differential Rotation of the Halo Traced by K-giant Stars

We use K-giant stars selected from the LAMOST DR5 to study the variation of the rotational velocity of the Galactic halo at different space positions. Modeling the rotational velocity distribution with both the halo and disk components, we find that the rotational velocity of the halo population decreases almost linearly with increasing vertical distance to the Galactic disk plane,Z, at fixed galactocentric radius,R. The samples are separated into two parts withZfor the two subsamples are 3.07 +/- 0.63 and -1.89 +/- 0.37 km s(-1) kpc(-1), respectively. Comparing with the TNG simulations, we suggest that this trend is caused by the interaction between the disk and halo. The results from the simulations show that only an oblate halo can provide a decreasing rotational velocity with increasing Z. This indicates that the Galactic halo is oblate with galactocentric radius R < 20 kpc. On the other hand, the flaring of the disk component (mainly the thick disk) is clearly traced by this study; with R between 12 and 20 kpc, the disk can vertically extend to 6 similar to 10 kpc above the disk plane. What is more interesting is that we find the Gaia-Enceladus-Sausage component has a significant contribution only in the halo with R < 12 kpc, i.e., a fraction of 23%-47%, while in the outer subsample, the contribution is too low to be well constrained.