SIMBAD references

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 ∼3.9 million stars within 1.5 kpc of the Sun, for which precise actions can be calculated from Gaia 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 ΔJ_R/ΔL_z and appear to be present in the stellar distribution out 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 |z| <= 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.