Comparison of ¹²CO channel maps at three different line-of-sight velocities (color maps) with predicted ellipses for parabolic WDS (white contours). The model used on each row is sketched in the left-most panel. Top row: classical model with radial Hubble expansion (see Eq. (8) and Lee et al. 2000). Middle and bottom rows: modified model with “collimated” expansion (see Eq. (11)). Green, blue, and yellow contours highlight specific height ranges, indicated in the first column. White dotted lines outline the radial boundary defined by the shell ellipses with increasing velocity. Model parameters are listed in Table 2. δx and δz units are arcseconds.