Trimodal structure of Hercules stream explained by originating from bar
  resonances
  Gaia Data Release 2 revealed detailed structures of nearby stars in phase
space. These include the Hercules stream, whose origin is still debated. Most
of the previous numerical studies conjectured that the observed structures
originate from orbits in resonance with the bar, based on static potential
models for the Milky Way. We, in contrast, approach the problem via a
self-consistent, dynamic, and morphologically well-resolved model, namely a
full $N$-body simulation of the Milky Way. Our simulation comprises about 5.1
billion particles in the galactic stellar bulge, bar, disk, and dark-matter
halo and is evolved to 10 Gyr. Our model's disk component is composed of 200
million particles, and its simulation snapshots are stored every 10 Myr,
enabling us to resolve and classify resonant orbits of representative samples
of stars. After choosing the Sun's position in the simulation, we compare the
distribution of stars in its neighborhood with Gaia's astrometric data, thereby
establishing the role of identified resonantly trapped stars in the formation
of Hercules-like structures. From our orbital spectral-analysis we identify
multiple, especially higher order resonances. Our results suggest that the
Hercules stream is dominated by the 4:1 and 5:1 outer Lindblad and corotation
resonances. In total, this yields a trimodal structure of the Hercules stream.
From the relation between resonances and ridges in phase space, our model
favored a slow pattern speed of the Milky-Way bar (40--45 $\mathrm{km \; s^{-1}
\; kpc^{-1}}$).
