Resonant Clumping and Substructure in Galactic Discs

TL;DR

This paper introduces a method to identify resonant orbits in galactic discs from N-body simulations, revealing how bar-driven resonances create observable kinematic substructures and bimodal velocity distributions in the outer disc.

Contribution

The paper presents a novel technique to extract resonant orbits in simulations, demonstrating their role in forming kinematic substructures and velocity bimodality in galactic discs.

Findings

Resonant orbits are present throughout the galactic disc, including outer regions.

Bar-driven resonances cause bimodal radial velocity distributions.

Transient overdensities in the disc appear and dissipate periodically.

Abstract

We describe a method to extract resonant orbits from N-body simulations exploiting the fact that they close in a frame rotating with a constant pattern speed. Our method is applied to the N-body simulation of the Milky Way by Shen et al. (2010). This simulation hosts a massive bar, which drives strong resonances and persistent angular momentum exchange. Resonant orbits are found throughout the disc, both close to the bar itself and out to the very edges of the disc. Using Fourier spectrograms, we demonstrate that the bar is driving kinematic substructure even in the very outer parts of the disc. We identify two major orbit families in the outskirts of the disc that make significant contributions to the kinematic landscape, namely the m:l = 3:-2 and 1:-1 families resonating with the pattern speed of the bar. A mechanism is described that produces bimodal distributions of Galactocentric radial velocities at selected azimuths in the outer disc. It occurs as a result of the temporal coherence of particles on the 3:-2 resonant orbits, which causes them to arrive simultaneously at pericentre or apocentre. This resonant clumping, due to the in-phase motion of the particles through their epicycle, leads to both inward and outward moving groups which belong to the same orbital family and consequently produce bimodal radial velocity distributions. This is a possible explanation of the bimodal velocity distributions observed towards the Galactic anti-Centre by Liu et al. (2012). Another consequence is that transient overdensities appear and dissipate (in a symmetric fashion) on timescales equal to the their epicyclic period resulting in a periodic pulsing of the disc's surface density.