paper

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

arXiv:1901.07568 · doi:10.1093/mnras/stz1875

Abstract

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 - plane, the undulations in the -- space and the streams in the - plane. We use a collisionless N-body simulation with co-spatial 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 - plane due to the OLR which translates to streams in the - 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 family are preferentially found at negative , giving rise to a `horn'-like feature, while orbits with larger libration amplitudes, trapped around the and families, constitute the positive 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 in the - plane (i.e. Hercules vs. `the horn') and subsequently to undulations in the -- space. We also explore a scenario in which the Sun is placed closer to the bar corotation and find that the bar perturbation cannot give rise to the these features.

17 pages, accepted for publication in MNRAS

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