Formation of the Two-armed Phase Spiral from Multiple External Perturbations
J. Lin, Zhao‐Yu Li, Rui Guo, John Douglas Hunt, T. Antoja, Chengye Cao
Abstract
Abstract Recent studies using the Gaia DR3 data have revealed a two-armed phase spiral in the Z – V Z phase space in the inner disk. In this study, we present new features of the two-armed phase spiral revealed by the Gaia data and a new mechanism to explain such features with multiple external perturbations. By segmenting the Gaia DR3 Radial Velocity Spectrometer (RVS) catalog based on J ϕ (or R g ) and θ ϕ , we confirm the existence of a clear two-armed phase spiral in the inner disk. Moreover, we identify a different two-armed phase spiral pattern at slightly larger radii, resembling a weak secondary branch along with the prominent major branch. At a given radius, with the azimuthal angle increasing, we observe a systematic transition of the two-armed phase spiral, with the significance of one branch weakened and another branch enhanced. This two-armed phase spiral may be due to the overlapping of distinct one-armed phase spirals. At different radii, the perturbation times estimated from each branch of the two-armed phase spiral are ∼320 and ∼500 Myr, respectively, suggesting that the Galactic disk could be impacted by double external perturbers separated by ∼180 Myr. We also performed test-particle simulations of the disk perturbed by two satellite galaxies, which successfully generated a two-armed phase spiral similar to the observation. Both the observation and simulation results suggest that the signature in the Z – V Z phase space of earlier perturbations may not be completely erased by the more recent one.