Phase Wrapping of Epicyclic Perturbations in the Wobbly Galaxy
  We use test-particle integrations to show that epicyclic motions excited by a
pericentre passage of a dwarf galaxy could account for bulk vertical velocity
streaming motions recently observed in the Galactic stellar disc near the Sun.
We use fixed potential test-particle integrations to isolate the role of phase
wrapping of epicyclic perturbations from bending and breathing waves or modes,
which require self-gravity to oscillate. Perturbations from a fairly massive
Sagittarius dwarf galaxy, $M_d \sim 2.5 \times 10^{10} M_\odot$, are required
to account for the size of the observed streaming motions from its orbital
pericentre approximately a Gyr ago. A previous passage of the dwarf through the
Galactic disc approximately 2.2 Gyr ago (with a then more massive dwarf galaxy)
is less effective. If phase wrapping of epicyclic perturbations is responsible
for stellar streaming motions in the Galactic disc, then there should be
variations in velocity gradients on scales of a few kpc in the vicinity of the
Sun.
