Turbulent velocity as a function of size. Black symbols show our data for CO, H₂1–0 lines, and Paα, respectively. The red dotted line is for α_vir = 1 for a constant mass-surface density Σ_gas = 250 M_⊙ pc^-2 and the green hatched region shows the range implied by Larson’s scaling law and the range of velocities measured on kpc scales. Black circles show the molecular clouds of Heyer et al. (2009). These points fall near the α_vir = 1 relation for gas surface densities  ~5 smaller than that of 3C 326 N. The horizontal dot-dashed line shows the maximal clump size of  ~100 pc. See Sect. 4 for details.