Comparisons between the velocity dispersions and column densities of HC₃N and H I. Left: cumulative distributions of velocity dispersions. Solid lines and dashed lines represent σ and σ^NT, respectively (Sect. 6.5). Right: correlation between the column densities of H I and those of HC₃N. Green and orange lines represent the linear fitting with and without the two data in blue shadowing, respectively. For data in the blue shadowing, the N(H I) parameters are not reliable with regard to the fit optical depth of HINSA τ < 0.01 (Sect. 5.2.5).