The echo of the bar buckling: phase-space spirals in Gaia DR2
  Using a single N-body simulation ($N=0.14\times 10^9$) we explore the
formation, evolution and spatial variation of the phase-space spirals similar
to those recently discovered by Antoja et al. in the Milky Way disk, with Gaia
DR2. For the first time in the literature, we use a self-consistent N-body
simulation of an isolated Milky Way-type galaxy to show that the phase-space
spirals develop naturally from vertical oscillations driven by the buckling of
the stellar bar. We claim that the physical mechanism standing behind the
observed incomplete phase-space mixing process can be internal and not
necessarily due to the perturbation induced by a massive satellite. In our
model, the bending oscillations propagate outwards and produce axisymmetric
variations of the mean vertical coordinate and of the vertical velocity
component. As a consequence, the phase-space wrapping results in the formation
of patterns with various morphology across the disk, depending on the bar
orientation, distance to the galactic center and time elapsed since the bar
buckling. Once bending waves appear, they are supported for a long time via
disk self-gravity. The underlying physical mechanism implies the link between
in-plane and vertical motion that leads directly to phase-space structures
whose amplitude and shape are in remarkable agreement with those of the
phase-space spirals observed in the Milky Way disk. In our isolated galaxy
simulation, phase-space spirals are still distinguishable, at the solar
neighbourhood, 3 Gyr after the buckling phase. The long-lived character of the
phase-space spirals generated by the bar buckling instability cast doubts on
the timing argument used so far to get back at the time of the onset of the
perturbation: phase-space spirals may have been caused by perturbations
originated several Gyrs ago, and not as recent as suggested so far.
