Position-velocity diagram unfolding the two velocity components of W43 into the ¹²CO 1–0 cloud ensembles #2 and #3 (arrows, see Sect. 3.3). The distributions of ¹²CO 1–0 (color) are plotted against the Galactic longitude, kept as the x axis, as in Fig. 2. The ¹²CO 1–0 lines have been summed over the b = −1° to b = 1° latitude range corresponding to the W43 complex and its H i envelope. The white boxes represent the longitude extent and velocity range of the W43 molecular complex (continuous lines) and its H i envelope (dashed lines). The thick gray curve indicates the theoretical location of the Scutum-Centaurus arm in the model by Vallée (2008).