Hα velocity dispersion, σ, as function of the star-formation intensity, Σ_SFR (M_⊙ yr^-1 kpc^-2). The lines represent the Toomre criterion for gas, assuming both Q = 1 and that the Schmidt-Kennicutt relation is appropriate for a variety of rotation speeds and radii, as in Fig. 13 and as indicated in the legend (see text for details). The underlying display of the normalized data (shown in various shades of red) is the same as in Fig. 9 (see that figure and text for details).