New features of the Gaia phase spiral and its possible relation to internal perturbations

Disc stars from the Gaia DR3 RVS sample are selected to explore the phase spiral as a function of position in the Galaxy. The data reveal a two-armed phase spiral pattern in the local $z-v_z$ plane inside the solar radius, which appears clearly when colour-coded by $\langle v_R \rangle (z,v_z)$: this is characteristic of a breathing mode that can in principle be produced by in-plane non-axisymmetric perturbations. We note that, on the contrary, the phase spiral pattern becomes single armed when outside the solar radius. When a realistic analytic model with an axisymmetric background potential plus a realistic steadily rotating central bar and 2-armed spiral arms as perturbation is used to perform particle test integrations, the pseudo stars get a prominent spiral pattern in the $\langle v_R \rangle$ map in the $x-y$ plane. Additionally, clear breathing mode evidence at a few $\rm{kms}^{-1}$ level can be seen in the $\langle v_z \rangle$ map on the $x-z$ plane, confirming that such breathing modes are non-negligible in the joint presence of a bar and spiral arms. However, no phase-spiral is perceptible in the $(z, v_z)$ plane. When an initial vertical perturbation is added to all pseudo stars and the same orbital integrating scheme is adopted to carry out the simulation, we find that phase spirals can clearly be seen 500~Myr after the perturbation and gradually disappear inside-out; in that case, they are, however, one-armed. Finally, we show as a proof of concept how a toy model of a time-varying non-axisymmetric in-plane perturbation with varying amplitude and pattern speed can, on the other hand, produce a strong two-armed phase-spiral. We conclude that a time-varying strong internal perturbation together with an external vertical perturbation could perhaps explain the transition between the two-armed and one-armed phase-spirals around the Solar radius.