On the ridges, undulations, and streams in Gaia DR2: linking the topography of phase space to the orbital structure of an N-body bar

We explore the origin of phase-space substructures revealed by the second Gala data release ill the disc of the Milky Way, such as the ridges in the V-phi-r plane, the undulations in the V-phi-r-V-r space and the streams in the V-phi-V-r plane. We use a collisionless N-body simulation with cospatial thin and thick discs, along with orbit integration, to study the orbital structure close to the Outer Lindblad Resonance (OLR) of the bar. We find that a prominent, long-lived ridge is formed in the V-phi-r plane due to the OLR which translates to streams in the V-phi-V-r plane and examine which closed periodic and trapped librating orbits are responsible for these features. We find that orbits which carry out small librations around the x(1)(1) family are preferentially found at negative V-r, giving rise to a 'horn' like feature, while orbits with larger libration amplitudes, trapped around the x(1)(2) and x(1)(1) families, constitute the positive V-r substructure, i.e. the Hercules-like feature. This changing libration amplitude of orbits will translate to a changing ratio of thin/thick disc stars, which could have implications on the metallicity distribution in this plane. We find that a scenario in which the Sun is placed close to the OLR gives rise to a strong asymmetry in V-r in the V-phi-V-r plane (i.e. Hercules versus 'the horn') and subsequently to undulations in the V-phi-r-V-r space. We also explore a scenario in which the Sun is placed closer to the bar corotation and find that the bar perturbation alone cannot give rise to these features.