# Mapping the Galactic disk with the LAMOST and Gaia Red clump sample: III: A new velocity substructure and time stamps of the Galactic disk asymmetry in the disk between 12$-$15 \,kpc

## TL;DR

This study maps the Milky Way's disk between 12-15 kpc, revealing a new velocity substructure and analyzing its temporal stability, suggesting in-plane mechanisms as the cause of observed asymmetries.

## Contribution

It introduces a detailed analysis of velocity asymmetries and substructures in the outer Galactic disk using LAMOST and Gaia data, with insights into their temporal evolution.

## Key findings

- Discovery of a velocity substructure associated with a density dip.
- Detection of vertical bulk motions without clear north-south asymmetry.
- Little temporal evolution of in-plane asymmetry over 14 Gyr.

## Abstract

We investigate the three-dimensional asymmetrical kinematics and present time stamps of the Milky Way disk between Galactocentric distances of $R=12$ and 15 \,kpc, using red clump stars selected from the LAMOST Galactic survey, also with proper motion measurements provided by the Gaia DR2. We discover velocity substructure above the Galactic plane corresponding to a density dip found recently ("south-middle opposite" density structure[R $\sim$ 12-15 \,kpc, Z $\sim$ 1.5 \,kpc] discovered in \citet{Wang2018b, Wang2018c}) in the radial and azimuthal velocity. For the vertical velocity, we 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, we 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. We propose that the possible scenario of this asymmetric velocity structure is caused by the mechanisms generated in-plane, rather than vertical perturbations.