A research team led by Dr. Hai-Feng Wang, LAMOST Fellow, in collaboration with Dr. Yang Huang, Bing-Qiu Chen of YunanUnivesity, Dr. Jeffrey Carlin of Arizona University, Dr.Martín López-Corredoiraof Instituto de Astrofísica de Canarias, and Prof. Li-Cai Deng of National Astronomical Observatories of Chinese Academy of Sciences (NAOC), discovered a new velocity substructure above the Galactic plane corresponding to a density dip found recently (“south-middle opposite” density structure discovered in Wang et al.in the radial and azimuthal velocity firstly.
For the vertical velocity, they detect clear vertical bulk motions or bending mode motions, which has no clear north-south asymmetry corresponding to the in-plane asymmetrical features. In the subsample of stars with different ages, they surprisingly find that there is little temporal evolution of the in-plane asymmetry from 0?14 Gyr, which means the structure is sensitive to the perturbations in almost cosmic time possibly. The paper is accepted by the Astrophysical Journal.
Fig.1 Edge-on views of the kinematics of the disk derived using the red clump giant sample, the green/red colors predominate bins both above and below the plane, illustrating that there is a clear bulk motions at all radii.
Disk oscillations out to 25 kpc were unveiled by Xu et al. The wave-like pattern with the help of star counts shows four over-densities, e.g. in the north at distances of about 2 kpc (North near), in the south at 4-6 kpc (South middle), from the Sun in the anti-center direction. Wang et al. detected rich substructures displayed in the residuals of stellar density. Among them, the substructures O14-1.5 and D14 + 2.0 show a north-south asymmetry, the first one corresponds to the south?middle structure with high confidence, and the second one corresponds to a new substructure located in the opposite side of the south middle region (named as “south middle opposite”).
Followed by this, during this work, not only do they confirm this new density substructure from another point of view, but also do they give the time stamps on it and find that the in-plane asymmetry is decoupled with the vertical asymmetry for this range possibly, which is different from the recent Gaia work in the neighborhood.
A basic assumption often used to interpret observations is that galaxies, in the past, are in equilibrium or stationary in the potential. This was shown to be an invalid assumption with the evidence for a Galactic North-South asymmetry in the number density and bulk velocity of solar neighborhood stars revealed by Widrow et al. These observations began what is known as Galactoseismology for the Milky Way.
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