The phase spiral in Gaia DR3

Abstract

We aim to study the phase spiral in the Milky Way (MW) with Gaia DR3. We used an edge detection algorithm to find the border of the phase spiral, allowing us to robustly quantify its shape at different positions and for different selections. We calculated the time of onset of the phase-mixing by determining the different turns of the phase spiral and using the vertical frequencies from commonly used MW potential models. We find that the phase spiral extends down to $-1.2$ kpc in height below the plane (about 3 to 5 scale heights of the thin disc) and beyond $\pm 50$ km/s in $V_Z$. We see a secondary branch mostly at positive vertical velocities when coloured by azimuthal velocity and in the counts projection. We also find complex variations of the phase spirals with angular momentum and azimuth. All these possibly provide evidence of multiple perturbations (from different times or from different perturbers) and/or of the complexity of the phase mixing process. We detect the phase spiral from 6 to 11 kpc from the Galactic centre and find signatures of vertical asymmetries 1-2 kpc beyond this range. We measure small but clear variations with azimuth. When we determine the phase mixing times from the phase spiral at different angular momenta and using the different spiral turns (at different $Z$) we obtain inconsistent times with systematic differences (times increasing with $|L_Z|$ and with $|Z|$). Our determinations are mostly in the range of [0.3-0.9] Gyr, with an average of 0.5 Gyr. The inconsistencies do not change when using different usual potential models, different stellar distances or frequencies for different kinetic temperatures. They could stem from the inconsistency of potential models with the true MW, and from too simple modelling, in particular neglecting self-gravity, not considering the multiple perturbations and the interference with other processes.

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