First panel: spectrogram of the m = 1 bending mode. Lines indicate characteristic frequencies: Ω (solid), ± v (dashed), Ω ± v, and Ω ± 2 v (dotted). Second panel: spectrogram of the m = 2 breathing mode. Solid and dashed lines indicate Ω and Ω ± κ/2, respectively. Third panel: spectrogram of the surface m = 2 density wave. Solid and dashed lines indicate Ω and Ω ± κ/2, respectively. The bending mode shows distinct branches aligned with the resonant curves of Ω ± v and Ω−2 v. The breathing mode and the density wave rotate at nearly the same pattern speeds, suggesting that the breathing mode is excited by the bar and the tidally induced arms.