Normalized 2-dimensional histogram of the frequency of occurrence in the data set of Hα line widths as function of star formation rate intensity. The set of points for each galaxy are normalized such that all of the galaxies have the same mean star-formation rate intensity (their distribution of the velocity dispersion in each galaxy is unaffected). The mean value was chosen to match the mean value of the star-formation intensity in the model with σ = (ϵΣ_SFR)^1/2 as indicated by the red  +  sign in the right panel of Fig. 8 (~0.5 M_⊙ yr^-1 kpc^-2). As in Fig. 2, the darkest regions represent the highest frequency of occurrence in the data set. The green  ×  and blue  +  symbols represent our simulated data after beam smearing, assuming that the two quantities plotted are related as σ = (ϵΣ_SFR)^1/2 and σ = σ_sim, respectively.