A wobbly and flared Milky Way disk revealed

The study updates our understanding of the disk.

Relying on LAMOST-Gaia data, a team of astronomers revealed the wobbly and flared nature of the Milky Way disk. The discovery by the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC), Shanghai Astronomical Observatory of CAS, and Nanjing University improve our understanding about our home galaxy.

The milky way disk is symmetric and flat on the whole. The stars within this disk rotate around the galactic center, with mean radial and vertical velocities zero.

Prof. Zhao Gang, the corresponding author of the work, said, “The huge number of spectra obtained by LAMOST and the high-precision astrometric data released by Gaia provides a golden opportunity for re-exploring the disk structure.”

The team selected ~490,000 K-type giants from LAMOST DR8 and Gaia EDR3 as tracers- they found the existence of vertical wobbles in the three-dimensional mean velocities within the spatial range Galactocentric radius of 5–15 kpc and three kpcs above and below the galactic plane.

The K-type giants below the plane rotate faster than those found above, indicating that inner disk stars migrate radially toward the outer disk. In contrast, outer disk stars exhibit alternate inward and outward radial motions, with velocities highly dependent on the vertical distance.

Also, the vertical motion of the outer disk is dominated by a contraction-like breathing mode and an upward bending mode. On the other hand, there is only a weak rarefaction-like breathing mode and a trivial downward bending mode in the inner disk.

By estimating the scale height of the disk with the measured velocity ellipsoid, the team revisited the nature of the disk flaring. They found that the scale height increased obviously with the increasing galactocentric radius, leading to a flaring feature.

Moreover, the north and south disks were found to have constant flaring strength. Similar flaring structures were also detected in some external galaxies, indicating that a flared disk could be a common phenomenon in disk galaxies.

Xue Xiangxiang, the second author of the paper, said, “This study provides high-quality clues to the galactic structure and evolution. Continuous observations will help to tell a more insightful story of the Milky Way.”

Journal Reference:
  1. Ping-Jie Ding et al., Vertical Structure of Galactic Disk Kinematics from LAMOST K Giants, The Astronomical Journal (2021). DOI: 10.3847/1538-3881/ac0892

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