C¹⁸O spectra across the main axis of G351 in the most quiescent northern part of the filament. Spectra are collapsed parallel to the main axis. Each spectrum is normalised to its peak intensity value for visualisation. The numbers in each panel indicate the distance from the spine of G351. The label 0′′ denotes the spine. The dashed line indicates a reference velocity of − 3.5 km s⁻¹ (see Fig. 5).