Evidence for Populations-dependent vertical motions and the Long-lived Non-Steady Lopsided Milky Way Warp

TL;DR

This study analyzes vertical motions of different stellar populations in the Milky Way to understand the warp's structure, evolution, and dependence on stellar age, revealing a long-lived, non-steady, and lopsided warp influenced by population age.

Contribution

It provides the first detailed analysis of population-dependent vertical motions and warp asymmetries, supporting the warp as a long-lived, evolving structure.

Findings

Younger stars show stronger warp amplitudes.

Warp exhibits asymmetry along azimuthal angle.

Vertical motions vary with Galactocentric distance.

Abstract

We present the Galactic disk vertical velocity analysis using OB type stars (OB), Red Clump stars (RC), and Main-Sequence-Turn-Off stars (MSTO) with different average age populations crossed matched with LAMOST DR5 and Gaia DR3. We reveal the vertical velocities of the three populations varies clearly with the Galactocentric distance ($R$) and the younger stellar population has stronger increasing trend in general. The bending and breathing modes indicated by the vertical motions are dependent on the populations and they are varying with spatial locations. These vertical motions may be due to the Galactic warp, or minor mergers, or non-equilibrium of the disk. Assuming the warp is the dominant component, we find that the warp amplitude ($\gamma$, $Z_\omega$) for OB (younger population) is larger than that for RC (medium population) and the later one is also larger than that for MSTO (older population), which is in agreement with other independent analyses of stellar density distribution, and supports the warp is long-lived, non-steady structure and has time evolution. This conclusion is robust whether or not the line-of-nodes $\phi_w$ is fixed or as a free parameter (with $\phi_w$ is around 3$-$8.5$^{\circ}$ as best fit). Furthermore, we find that warp is lopsided with asymmetries along azimuthal angle ($\phi$).