Illustration of the distance prior based on the fitted linewidth. The red line shows the size-linewidth relation from Solomon et al. (1987) corrected for the most recent distance estimates to the Galactic centre. Red-shaded areas show the , , and intervals determined from variations of the parameters of the size-linewidth relation. The two dots show expected velocity dispersion values for corresponding physical extents of the beam for the near (white dot) and far (black dot) kinematic distance solutions. Blue horizontal lines show velocity dispersion values of fit components and dashed vertical lines indicate the relevant distances to the expected values, from which the prior is determined. See Sect. 3.3 for more details.