Peculiar in-plane velocities in the outer disc of the Milky Way

Abstract

We present the peculiar in-plane velocities derived from LAMOST red clump stars, which are purified and separated by a novel approach into two groups with different ages. The samples are mostly contributed around the Galactic anti-center direction so that we are able to map the radial profiles of the radial and azimuthal velocities in the outer disc. From variations of the in-plane velocities with Galactocentric radius for the younger and older populations, we find that both radial and azimuthal velocities are not axisymmetric at . The two red clump populations show that the mean radial velocity is negative within and positive beyond. This is likely because of the perturbation induced by the rotating bar. The cross-zero radius, , essentially indicates the rough location of the Outer Lindblad Resonance radius. Given the circular speed of 238 km s−1, the pattern speed of the bar can be approximated as 45 km s−1 kpc−1. The young red clump stars show larger mean radial velocity than the old population by about 3 km s−1 between and 12 kpc. This is possibly because the younger population is more sensitive to the perturbation than the older one. The radial profiles of the mean azimuthal velocity for the two populations show an interesting U-shape, i.e. at , the azimuthal velocity declines with R by about 10 km s−1, while at it increases with R to 240 – 245 km s−1. It is not clear why the mean azimuthal velocity shows this U-shape along the Galactic anti-center direction. Moreover, the azimuthal velocity for the younger population is slightly larger than that for the older one and the difference moderately declines with R. Beyond , the azimuthal velocities for the two populations are indistinguishable.