The Galactic disc in action space as seen by Gaia DR2

The quality and quantity of 6D stellar position-velocity measurements in the second Gaia data release's radial velocity sample (DR2/RVS) allow us to study small-scale structure in the orbit distribution of the Galactic disc beyond the immediate Solar neighbourhood. We investigate the distribution of orbital actions (J(R), L-z, J(z)) of similar to 3.9 million stars within 1.5 kpc of the Sun, for which precise actions can be calculated from Gala DR2 alone. In the n(J(R), L-z) distribution, we identify at least seven stellar overdensities at high J(R) that lie along lines of constant slope Delta J(R)/Delta L-z and appear to be present in the stellar distribution out to similar to 1.5 kpc at the same location in (L-z, J(R)). The known moving groups in the (U, V) plane of the Solar neighbourhood are local manifestations of this extended system of orbit substructure at low J(R), as expected. The n(J(R), L-z) features are most prominent among stars within vertical bar z vertical bar less than or similar to 500 pc from the Galactic plane and with lower than average J(z). These features frequently coincide with (L-z, J(R)) regions of dramatic imbalance between stars moving in and out, suggesting that stars are not phase-mixed along orbits or on resonant orbits. Some of these n(J(R), L-z) ridges resemble features expected from rapid orbit diffusion along particular (J(R), L-z) directions in the presence of various resonances. Orbital action and angle space of stars in Gaia DR2 is therefore highly structured over kpc scales, and appears to be very informative for modelling studies of non-axisymmetric structure and resonances in the Galactic disc.