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

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 8 〈 R 〈 14 kpc. The two red clump populations show that the mean radial velocity is negative within R - 9 kpc and positive beyond. This is likely because of the perturbation induced by the rotating bar. The cross-zero radius, R -9 kpc, 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 3km s^-1 between R-9 and 12kpc. 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 R 〈 10.Skpc, the azimuthal velocity declines with R by about 10km s^-1, while at R 〉 10.5 kpc 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 R0-12 kpc, the azimuthal velocities for the two populations are indistinguishable.

Red Clump Stars from LAMOST Ⅱ： the outer disc of the Milky Way

We present stellar density maps of the Galactic outer disc with red clump stars from LAMOST data.These samples are separated into younger(mean age^2.7 Gyr)and older(mean age^4.6 Gyr)populations so that they can trace the variation of structures with ages in the range of Galactocentric radius R from 9 to 13.5 kpc.We show that scale heights for both of the two populations increase with R and display radial gradients of 48±6 and 40±4 pc kpc^(-1)for the older and younger populations,respectively.It is evident that flaring occurs in the thin disc populations with a wide range of ages.Moreover,the intensity of flaring does not seem to be significantly related to the age of the thin disc populations.On the other hand,scale lengths of the radial surface density profiles are 4.7±0.5 kpc for the younger population and 3.4±0.2 kpc for the older population,meaning that the younger disc population is more radially extended than the older one.Although the fraction of the younger population mildly increases from 28% at R^9 to about 35% at R^13 kpc,the older population is prominent with a fraction of no less than 65% in the outer disc.

A Method of Obtaining the Pitch Angle of Spiral Arms and the Inclination of Galactic Discs

We investigate the mathematical form, the symmetry of spiral structure and the projected images of galactic discs. The measured pitch angles of spiral arms and inclination angles of galactic discs for 60 spiral galaxies are presented. The global spiral structure is emphasized in the study. It is found that, except for small-scale distortions, the spiral arms of those galaxies that were classified as AC 12 in the arm classification system of Elmegreen & Elmegreen, can be represented by the logarithmic spiral form.