(a) Face-on gas surface-density map at t = 180 Myr with the orbit of the representative particle that reaches the CND (red curve). The x-axis aligns with the bar major axis, and the bar rotates counterclockwise. (b) Slice along the line connecting the end points of the orange square in panel (a). The horizontal and vertical axes correspond to the direction along the gas flow and perpendicular to the disk, respectively. The z-direction is vertically exaggerated. An animation of the face-on surface-density map and the slice showing the gas flow perpendicular to the disk is available in the online journal. The animation proceeds from t = 150–180 Myr, tracing the orbit of the representative gas particle that finally forms CND. It represents how the vaulting motion relates with the 2D motion.

(An animation of this figure is available in the online article.)

(An animation of this figure is available.)

The video/animation of this figure is available in the online journal.