Comparison of the column density-second moment distribution of a model galaxy built to resemble NGC 1566 with that of NGC 1566 itself. The observed distribution is shown in the left panel; the model in the right panel. For the model we use a single, thin H I disk with a velocity dispersion of 10 km s⁻¹. The model galaxy is “observed” and treated just like the single-track MHONGOOSE galaxies. Note the plume due to beam smearing at the high column density end, and the masking tail at the low column density end. The model does not show the high second-moment values at intermediate column densities that are present in the observations.