Histogram showing the relative frequency of the centroid velocities of all fitted velocity components (PdBI+30 m CO(2−1) data). Velocities measured in the north and south are shown as gray dashed and solid lines, respectively, while the solid black line takes all components into consideration. The centroid velocity is shown transformed into the galaxy-plane (Eq. (1)). Overlaid rectangles represent the different predictions of the models described in Sect. 4. Two predictions for the bar are shown: the blue rectangle shows in Eq. (3) from scenario 1, in which the bar has a higher pattern speed than the spiral Ω_b> Ω_s, while the green rectangle shows in Eq. (6) from scenario 2, in which the bar and spiral have the same pattern speed Ω_b = Ω_s. The red rectangle shows the prediction for the spiral motions corresponding to in Eq. (8). Velocities spanned by the spiral illustrate a range of spiral arm strengths corresponding to Σ_a/ Σ₀ = 0.5−1.5.