We explore the kinematics (both the radial velocity and the proper motion) of
the vertical X-shaped feature in the Milky Way with an N-body bar/bulge model.
From the solar perspective, the distance distribution of particles is
double-peaked in fields passing through the X-shape. The separation and
amplitude ratio between the two peaks qualitatively match the observed trends
towards the Galactic bulge. We confirm clear signatures of cylindrical rotation
in the pattern of mean radial velocity across the bar/bulge region. We also
find possible imprints of coherent orbital motion inside the bar structure in
the radial velocity distribution along l=0 degree, where the near and far sides
of the bar/bulge show excesses of approaching and receding particles. The
coherent orbital motion is also reflected in the slight displacement of the
zero-velocity-line in the mean radial velocity, and the displacement of the
maximum/minimum in the mean longitudinal proper motion across the bulge region.
We find some degree of anisotropy in the stellar velocity within the X-shape,
but the underlying orbital family of the X-shape cannot be clearly
distinguished. Two potential applications of the X-shape in previous literature
are tested, i.e., bulge rotation and Galactic center measurements. We find that
the proper motion difference between the two sides of the X-shape can be used
to estimate the mean azimuthal streaming motion of the bulge, but not the
pattern speed of the bar. We also demonstrate that the Galactic center can be
located with the X-shape, but the accuracy depends on the fitting scheme, the
number of fields, and their latitudinal coverage.Comment: Minor changes to match the ApJ accepted version; 17 pages; emulateapj
format. The electronic tables of our model result are available upon reques
