Structure in phase space associated with spiral and bar density waves in an N-body hybrid galactic disc

An N-body hybrid simulation, integrating both massive and tracer particles, of a Galactic disc is used to study the stellar phase-space distribution or velocity distributions in different local neighbourhoods. Pattern speeds identified in Fourier spectrograms suggest that two-and three-armed spiral density waves, a bar and a lopsided motion are coupled in this simulation, with resonances of one pattern lying near resonances of other patterns. We construct radial and tangential (uv) velocity distributions from particles in different local neighbourhoods. More than one clump is common in these local velocity distributions regardless of the position in the disc. Features in the velocity distribution observed at one galactic radius are also seen in nearby neighbourhoods (at larger and smaller radii) but with shifted mean v values. This is expected if the v velocity component of a clump sets the mean orbital galactic radius of its stars. We find that gaps in the velocity distribution are associated with the radii of kinks or discontinuities in the spiral arms. These gaps also seem to be associated with Lindblad resonances with spiral density waves and so denote boundaries between different dominant patterns in the disc. We discuss implications for interpretations of the Milky Way disc based on local velocity distributions. Velocity distributions created from regions just outside the bar's outer Lindblad resonance and with the bar oriented at 45 from the Sun-Galactic centre line more closely resemble that seen in the solar neighbourhood (triangular in shape at lower uv and with a Hercules-like stream) when there is a strong nearby spiral arm, consistent with the observed Centaurus Arm tangent, just interior to the solar neighbourhood.