Top panel: distribution of dust brightness from the Hi-GAL-500 μm map (with 36′′ resolution, comparable with that of the line data) as a function of position along the main spine of G351. The starting point for the skeleton is the position α(J2000) = 17^h 26^m17.′′2, δ(J2000) = −36°02′56.′′7. Labels P1–P4 denote the four peaks detected in the Herschel 500 μm dust brightness and correspond to Clump-5, Clump-2, Clump-1, and Clump-3, respectively. Middle panel: velocity centroid for C¹⁸O (2–1) as a function of position along the skeleton of G351. The black squares represent the data points, the red curve obtained by resampling the data over a bin of 15′′ (equivalent to 0.07 pc). Botton panel: absolute velocity gradient along the spine computed over 0.07 pc.