Radial Wave in the Galactic Disk: New Clues to Discriminate Different Perturbations

Decoding the key dynamical processes that shape the Galactic disk structure is crucial for reconstructing the Milky Way's evolution history. The second Gaia data release unveils a novel wave pattern in the LZ- VR space, but its formation mechanism remains elusive due to the intricate nature of involved perturbations and the challenges in disentangling their effects. Utilizing the latest Gaia DR3 data, we find that the LZ- VR wave systematically shifts towards lower LZ for dynamically hotter stars. The amplitude of this phase shift between stars of different dynamical hotness (Δ LZ) peaks around 2300\,km\,s⁻¹\,kpc. To differentiate the role of different perturbations, we perform three sets of test particle simulations, wherein a satellite galaxy, corotating transient spiral arms, and a bar plus the corotating transient spiral arms act as the sole perturber, respectively. Under the satellite impact, the phase shift amplitude decreases towards higher LZ, which we interpret through a toy model of radial phase mixing. While the corotating transient spiral arms do not generate an azimuthally universal phase shift variation pattern, combining the bar and spirals generates a characteristic Δ LZ peak at 2:1 Outer Lindblad Resonance, qualitatively resembling the observed feature. Therefore, the LZ- VR is more likely of internal origin. Furthermore, linking the Δ LZ peak to the 2:1 Lindblad resonance offers a novel approach to estimating the pattern speed of the Galactic Bar, supporting the long/slow bar model.