At the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC), astronomers have disclosed a wobbly, flared Milky Way disk using LAMOST-Gaia data, which updates one’s knowledge about the disk.
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The study was performed in collaboration with the Shanghai Astronomical Observatory of CAS and Nanjing University. The findings of the study have been published in The Astronomical Journal.
The Milky Way is known as a typical disk galaxy. According to the classical view, the Milky Way’s disk is symmetric and flat on the whole, similar to a pancake. Stars present in the disk rotate around the Galactic center, with zero mean radial and vertical velocities.
Having gathered huge observational data offered by large survey projects over the past few years, more details concealed in the Milky Way disk have turned out to be evident, casting doubts on the conventional imagination regarding the galaxy.
The huge number of spectra obtained by LAMOST and the high-precision astrometric data released by Gaia provide a golden opportunity for re-exploring the disk structure.
Gang Zhao, Study Corresponding Author and Professor, National Astronomical Observatories of China
Headed by Dr. Pingjie Ding, the study’s first author, the scientists chose approximately 490,000 K-type giants from LAMOST DR8 and Gaia EDR3 as tracers. They discovered that Vertical wobbles occurred in the three-dimensional mean velocities in the spatial range of the Galactocentric radius of 5-15 kpc and 3 kpc above and below the Galactic plane.
In contrast to the classical view that stellar motions are symmetric with respect to the Galactic plane, the K-type giants below the plane are discovered to rotate faster compared to those above. At the same time, inner disk stars tend to migrate radially toward the outer disk, while outer disk stars display alternate inward and outward radial motions, with velocities highly dependent on the vertical distance.
Furthermore, a contraction-like breathing mode and an upward bending mode tend to dominate the outer disk’s vertical motion. By contrast, there is only a weak rarefaction-like trivial downward bending mode and breathing mode in the inner disk.
Then, the researchers went on to revisit the nature of the disk flaring by evaluating the disk’s scale height with the quantified velocity ellipsoid. They discovered that the scale height rose obviously with the growing Galactocentric radius, resulting in a flaring feature.
Moreover, the outcomes denoted that the north and south disks possessed consistent flaring strength. Also, similar flaring structures were detected in few external galaxies, implying that a flared disk could be a general phenomenon taking place in disk galaxies.
The wobbly disk that has been probed by the LAMOST-Gaia K-type giants can be attributed to the perturbations generated by both in-plane substructures (such as the spiral arms and the central bar) and long-lived external perturbers (like a satellite galaxy). Yet, the mechanism behind the flaring structure remains puzzling.
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.
Xiangxiang Xue, Study Second Author, National Astronomical Observatories of China
Journal Reference
Ding, P.-J., et al. (2021) Vertical Structure of Galactic Disk Kinematics from LAMOST K Giants. The Astronomical Journal. doi.org/10.3847/1538-3881/ac0892.