The Vertical X-shaped Structure in the Milky Way: Evidence from a Simple
  Boxy Bulge Model
  A vertical X-shaped structure was recently reported in the Galactic bulge.
Here we present evidence of a similar X-shaped structure in the Shen et al.
(2010) bar/boxy bulge model that simultaneously matches the stellar kinematics
successfully. The X-shaped structure is found in the central region of our
bar/boxy bulge model, and is qualitatively consistent with the observed one in
many aspects. End-to-end separations of the X-shaped structure in the radial
and vertical directions are roughly 3 kpc and 1.8 kpc, respectively. The
X-shaped structure contains about 7% of light in the boxy bulge region, but it
is significant enough to be identified in observations. An X-shaped structure
naturally arises in the formation of bar/boxy bulges, and is mainly associated
with orbits trapped around the vertically-extended x_1 family. Like the bar in
our model, the X-shaped structure tilts away from the Sun--Galactic center line
by 20 degrees. The X-shaped structure becomes increasingly symmetric about the
disk plane, so the observed symmetry may indicate that it formed at least a few
billion years ago. The existence of the vertical X-shaped structure suggests
that the formation of the Milky Way bulge is shaped mainly by internal disk
dynamical instabilities.
