On the ridges, undulations, and streams in Gaia DR2: linking the topography of phase space to the orbital structure of an N-body bar. Issue 3 (10th July 2019)

• Record Type: Journal Article
• Authors: Fragkoudi, F; Katz, D; Trick, W; White, S D M; Di Matteo, P; Sormani, M C; Khoperskov, S; Haywood, M; Hallé, A; Gómez, A
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 488:Issue 3(2019)
• Page Start: 3324
• Page End: 3339
• Publication Date: 2019-07-10
• Subjects: galaxies: kinematics and dynamics -- Galaxy: disc -- Galaxy: evolution -- Galaxy: kinematics and dynamics; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stz1875
• Languages: English
• ISSNs: 0035-8711

Abstract

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 V ϕ – r plane, the undulations in the V ϕ – r – Vr space and the streams in the V ϕ – Vr 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 ϕ – r plane due to the OLR which translates to streams in the V ϕ – Vr 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 Vr, 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 Vr 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 Vr in the V ϕ – Vr plane (i.e. Hercules versus 'the horn') and subsequently to undulations in the V ϕ – r – Vr 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.