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

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-V r space and the streams in the V φ -V r 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 V φ -r plane due to the OLR which translates to streams in the V φ -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 φ -V r plane (i.e. Hercules vs. 'the horn') and subsequently to undulations in the V φ -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 the these features.

Fluctuations in galactic bar parameters due to bar–spiral interaction

“…the uv plane). Due to the large number of particles required, typically such studies are done with test-particle integrations (Dehnen 2000;Fragkoudi et al 2019) or analytically (Monari et al 2017a(Monari et al , 2019, in which case the bar is in a steady state. What creates the structure in the uv plane are the locations of resonances in the disc, which are set by the pattern speed.…”

Identifying resonances of the Galactic bar in Gaia DR2: I. Clues from action space

“…In the Fiducial_40 simulation, we can therefore determine the dominant orbital frequencies Ω ,true and Ω ,true from the maximum peak of the Fourier transform of the star's time evolution in ( ) and cos ( ) (e.g. Binney & Spergel 1982;Laskar 1993;Fragkoudi et al 2019) with a precision of 2 /( int ) = 2 /(50 bar ) = 0.8 km/s/kpc. We select stars in the simulation that are (not in the axisymmetric estimate but) actually in the barred system in resonance with the bar's OLR ( = 1, = 2) using the resonance condition | Ω bar − Ω ,true − × Ω ,true | < 0.8 km/s/kpc, and Ω ,true > 15 km/s/kpc.…”

Chemodynamics of barred galaxies in cosmological simulations: On the Milky Way’s quiescent merger history and in-situ bulge

“…Some of the most striking features the data have revealed are the prominent ridges in V φ −r space (Kawata et al 2018;Antoja et al 2018), which have undulations in V r associated to them (Fragkoudi et al 2019). The bar has been proposed as a culprit for a number of these features including the observed ridges in the V φ −r plane (Fragkoudi et al 2019) and the Gaia spiral via the buckling instability (Khoperskov et al 2019; but see Laporte et al 2019 for an alternative explanation). Furthermore, as shown recently by Khanna et al (2019), the ridges exhibit different abundance trends compared to phase space around them, which could perhaps give clues as to their origin.…”

“…Gaia DR2 has allowed for a detailed exploration of phase-space of the Milky Way's disc, revealing a number of previously unknown substructures, such as the Gaia snail or spiral (Antoja et al 2018). Some of the most striking features the data have revealed are the prominent ridges in V φ −r space (Kawata et al 2018;Antoja et al 2018), which have undulations in V r associated to them (Fragkoudi et al 2019). The bar has been proposed as a culprit for a number of these features including the observed ridges in the V φ −r plane (Fragkoudi et al 2019) and the Gaia spiral via the buckling instability (Khoperskov et al 2019; but see Laporte et al 2019 for an alternative explanation).…”