Resonant Orbits and the High Velocity Peaks Towards the Bulge

We extract the resonant orbits from an N-body bar that is a good representation of the Milky Way, using the method recently introduced by Molloy et al. (2015). By decomposing the bar into its constituent orbit families, we show that they are intimately connected to the boxy-peanut shape of the density. We highlight the imprint due solely to resonant orbits on the kinematic landscape towards the Galactic centre. The resonant orbits are shown to have distinct kinematic features and may be used to explain the cold velocity peak seen in the APOGEE commissioning data (Nidever at al., 2012). We show that high velocity peaks are a natural consequence of the motions of stars in the 2:1 orbit family and that stars on other higher order resonances can contribute to the peaks. The locations of the peaks vary with bar angle and, with the tacit assumption that the observed peaks are due to the 2:1 family, we find that the locations of the high velocity peaks correspond to bar angles in the range 10 < theta_bar < 25 (deg). However, some important questions about the nature of the peaks remain, such as their apparent absence in other surveys of the Bulge and the deviations from symmetry between equivalent fields in the north and south. We show that the absence of a peak in surveys at higher latitudes is likely due to the combination of a less prominent peak and a lower number density of bar supporting orbits at these latitudes.

Resonant Clumping and Substructure in Galactic Discs

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 discs surface density.