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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 kinematicssuccessfully. The X-shaped structure is found in the central region of ourbar/boxy bulge model, and is qualitatively consistent with the observed one inmany aspects. End-to-end separations of the X-shaped structure in the radialand vertical directions are roughly 3 kpc and 1.8 kpc, respectively. TheX-shaped structure contains about 7% of light in the boxy bulge region, but itis significant enough to be identified in observations. An X-shaped structurenaturally arises in the formation of bar/boxy bulges, and is mainly associatedwith orbits trapped around the vertically-extended x_1 family. Like the bar inour model, the X-shaped structure tilts away from the Sun--Galactic center lineby 20 degrees. The X-shaped structure becomes increasingly symmetric about thedisk plane, so the observed symmetry may indicate that it formed at least a fewbillion years ago. The existence of the vertical X-shaped structure suggeststhat the formation of the Milky Way bulge is shaped mainly by internal diskdynamical instabilities.

The Vertical X-shaped Structure in the Milky Way: Evidence from a Simple Boxy Bulge Model