New phase space structures in the Gaia data: a dynamically young disk

We will present the new phase space substructures sampled by the six million stars’ dataset with 6D phase-space information from the Gaia satellite, thanks to the large amount of data and their excellent precision. The new morphologies discovered are a snail-shaped structure in the combination of vertical position and velocity, and diagonal ridges in the azimuthal velocity combined with galactic radius. The vertical structures are signatures of ongoing phase-mixing and our hypothesis is that the Milky Way disk might have been perturbed in the past. This helps us to determine properties of the possible perturbation and parameters of the Milky Way potential as well. By considering a realistic model for the potential of the Galaxy and the associated orbital vertical frequencies, we infer that a perturbation might have occurred between 300 and 900 million years ago, which is consistent with estimates of the previous pericenter of the Sagittarius dwarf galaxy. But we will also review alternative hypothesis presented so far such as the effects of the buckling of the bar or bending waves in the disk. The in-plane diagonal ridges might be a result of phase-mixing or/and resonant effects of the bar and spiral arms. In any case, the Gaia data prove that the disk of our Galaxy strongly changes with time and is very responsive to perturbations.