The Galactic disc in action space as seen byGaiaDR2

The quality and quantity of 6D stellar position-velocity measurements in the second Gaia data release (DR2) allows to study small-scale structure in the orbit distribution of the Galactic disc beyond the immediate Solar neighborhood. We investigate the distribution of orbital actions (JR, J_=Lᵦ, Jᵦ) of 3. 5 million stars within 1. 5~kpc of the Sun, for which precise actions can be calculated from Gaia DR2 alone. This distribution n (JR, Lᵦ) reveals a remarkable amount of sub-structure. The known moving groups in the (U, V) -plane of the Solar neighborhood correspond to overdensities in (JR, Lᵦ), as expected. But n (JR, Lᵦ) also exhibits a wealth of density clumps and ridges that extend towards higher JR. These n (JR, Lᵦ) features are most prominent among orbits that stay close to the Galactic plane and remain consistently visible out to 1. 5~kpc, as opposed to the sub-structure in velocity space. Some of these n (JR, Lᵦ) ridges resemble features expected from rapid orbit diffusion along particular (JR, Lᵦ) -directions in the presence of various resonances. Several of these n (JR, Lᵦ) structures show a dramatic imbalance of stars moving in or out, suggesting that stars are not phase-mixed along orbits or on resonant orbits. Orbital action and angle space of stars in Gaia DR2 is therefore highly structured over kpc-scales, and appears to be very informative for modeling studies of non-axisymmetric structure and resonances in the Galactic disc.