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

Abstract

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 $L_Z-\langle V_R\rangle$ 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 $L_Z-\langle V_R\rangle$ wave systematically shifts toward lower $L_Z$ for dynamically hotter stars with larger $J_Z$ values. The amplitude of this phase shift between stars of different dynamical hotness ($\Delta L_Z$) peaks at around $\mathrm{2100\,km\,s^{-1}\,kpc}$. To differentiate the role of different perturbations, we perform three sets of test particle simulations, wherein a satellite galaxy, transient spiral arms, and a bar plus the transient spiral arms act as the sole perturber, respectively. Under the satellite impact, the phase shift amplitude $\Delta L_Z$ decreases toward higher $L_Z$, which we interpret through a toy model of radial phase mixing. While neither the transient spiral arms nor the bar generates an azimuthally universal phase shift variation pattern, combining the bar and spirals generates a characteristic $\Delta L_Z$ peak at the 2:1 Outer Lindblad Resonance (OLR) of the bar, qualitatively resembling the observed feature. Therefore, the $L_Z-\langle V_R\rangle$ wave is more likely of internal origin. Furthermore, linking the $\Delta L_Z$ peak to the 2:1 OLR offers a novel approach to constraining the pattern speed of the Galactic bar, supporting the long/slow bar model.