2019MNRAS.488.3324F

We explore the origin of phase-space substructures revealed by the second Gaia data release in the disc of the Milky Way, such as the ridges in the V_ph-r plane, the undulations in the V_ph-r-V_r space and the streams in the V_ph-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_ph-r plane due to the OLR which translates to streams in the V_ph-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 x₁(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 x₁(2) and x₁(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_ph-V_r plane (i.e. Hercules versus 'the horn') and subsequently to undulations in the V_ph-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.