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The outer parts of the Milky Way's disc are significantly out of equilibrium. Using only distances and proper motions of stars from Gaia 's Early Data Release 3, in the range | b | < 10°, 130° < ℓ < 230°, we show that for stars in the disc between around 10 and |$14\, \mathrm{kpc}$| from the Galactic centre, vertical velocity is strongly dependent on the angular momentum, azimuth, and position above or below the Galactic plane. We further show how this behaviour translates into a bimodality in the velocity distribution of stars in the outer Milky Way disc. We use an N -body model of an impulse-like interaction of the Milky Way disc with a perturber similar to the Sagittarius dwarf to demonstrate that this mechanism can generate a similar disturbance. It has already been shown that this interaction can produce a phase spiral similar to that seen in the Solar neighbourhood. We argue that the details of this substructure in the outer galaxy will be highly sensitive to the timing of the perturbation or the gravitational potential of the Galaxy, and therefore may be key to disentangling the history and structure of the Milky Way. [ABSTRACT FROM AUTHOR]

Gaia DR2 has revealed new small-scale and large-scale patterns in the phase-space distribution of stars in the Milky Way. In cylindrical Galactic coordinates |$(R,\phi ,z)$|⁠ , ridge-like structures can be seen in the |$(R,V_\phi)$| plane and asymmetric arch-like structures in the |$(V_R,V_\phi)$| plane. We show that the ridges are also clearly present when the third dimension of the |$(R,V_\phi)$| plane is represented by |$\langle z \rangle$|⁠ , |$\langle V_z \rangle$|⁠ , |$\langle V_R \rangle$|⁠ , |$\langle$| [Fe/H] |$\rangle$|⁠ , and |$\langle [\alpha /{\rm Fe}]\rangle$|⁠. The maps suggest that stars along the ridges lie preferentially close to the Galactic mid-plane (⁠|$|z|\lt 0.2$| kpc), and have metallicity and |$\alpha$| elemental abundance similar to that of the Sun. We show that phase mixing of disrupting spiral arms can generate both the ridges and the arches. It also generates discrete groupings in orbital energy – the ridges and arches are simply surfaces of constant energy. We identify eight distinct ridges in the Gaia DR2 data: six of them have constant energy while two have constant angular momentum. Given that the signature is strongest for stars close to the plane, the presence of ridges in |$\langle z \rangle$| and |$\langle V_z \rangle$| suggests a coupling between planar and vertical directions. We demonstrate, using N-body simulations that such coupling can be generated both in isolated discs and in discs perturbed by an orbiting satellite like the Sagittarius dwarf galaxy. [ABSTRACT FROM AUTHOR]

We use the second data releases of the European Space Agency Gaia astrometric survey and the high-resolution Galactic Archaeology with HERMES (GALAH) spectroscopic survey to analyse the structure of our Galaxy's disc components. With GALAH, we separate the α-rich and α-poor discs (with respect to Fe), which are superposed in both position and velocity space, and examine their distributions in action space. We study the distribution of stars in the zV |$\mathrm{ z}$| phase plane, for both V ϕ and VR, and recover the remarkable 'phase spiral' discovered by Gaia. We identify the anticipated quadrupole signature in zV |$\mathrm{ z}$| of a tilted velocity ellipsoid for stars above and below the Galactic plane. By connecting our work with earlier studies, we show that the phase spiral is likely to extend well beyond the narrow solar neighbourhood cylinder in which it was found. The phase spiral is a signature of corrugated waves that propagate through the disc, and the associated non-equilibrium phase mixing. The radially asymmetric distribution of stars involved in the phase spiral reveals that the corrugation, which is mostly confined to the α-poor disc, grows in z -amplitude with increasing radius. We present new simulations of tidal disturbance of the Galactic disc by the Sagittarius (Sgr) dwarf. The effect on the zV |$\mathrm{ z}$| phase plane lasts |${\gtrsim } 2\, \mathrm{Gyr}$|⁠, but a subsequent disc crossing wipes out the coherent structure. We find that the phase spiral was excited |${\lesssim } 0.5\, \mathrm{Gyr}$| ago by an object like Sgr with total mass ∼3 × 1010 M⊙ (stripped down from ∼5 × 1010 M⊙ when it first entered the halo) passing through the plane. [ABSTRACT FROM AUTHOR]