Unstable perturbations in the M 33 disk. In the upper panel the long- and short-dashed lines show the most unstable wavelength according to Jeans criteria and to Toomre criteria, respectively, as a function of galactocentric radius. The continuous line indicates twice the vertical scale height for the gas. In the bottom panel the predicted molecular cloud masses for perturbations with radius equal to the vertical gaseous disk scale height are shown with a continuous line as a function of galactocentric radius. The long-dashed line indicates the mass corresponding to perturbations with radius equal to one-quarter the Jeans length.