Gaia DR3 features of the phase spiral and its possible relation to internal perturbations

Abstract

Disc stars from the Gaia DR3 RVS catalogue are selected to explore the phase spiral 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 ⟨v_R⟩(z, v_z): this is characteristic of a breathing mode that can in principle be produced by in-plane non-axisymmetric perturbations. The phase spiral pattern becomes single armed outside the solar radius. When a realistic analytic model with a steadily rotating 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 ⟨v_R⟩ map in the x-y plane. Additionally, clear breathing mode evidence at a few $\, \mathrm{km\, s}^{-1}$ level can be seen in the ⟨v_z⟩ 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 carry out the simulation, the one-armed phase spirals can clearly be seen 500 Myr after the perturbation and gradually disappear inside out. Finally, we show as a proof of concept how a toy model of a time-varying non-axisymmetric in-plane perturbation with varying pattern speed can produce a strong two-armed phase spiral. We conclude 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.