Specific star formation rate (sSFR, in Gyr^-1) as a function of stellar mass surface density (Σ_⋆, in M_⊙ pc^-2), for the hypothesis that the star formation drives galaxies towards the line of stability against star formation (Toomre parameter, Q = 1) as described in the text. The red and blue solid lines represents the sSFR where the energy input from massive stars generates enough turbulence to drive the global ISM towards Q ~ 1. The red solid lines represent where Q ~ 1 for a range of gas fractions (f_g = 0.3 to 0.7) and for a constant σ_gas/σ_stars = 1, while the blue lines represent where Q = 1 for a constant gas fraction (f_g = 0.5) and σ_gas/σ_stars = 1, but now for a range of energy input from massive stars into the turbulence of the ISM, ϵ = 100–160 km s^-1 kpc yr, where ϵ is the coupling between the star formation intensity and the velocity dispersion (Lehnert et al. 2009, 2013). The predicted sSFR values increase with increasing gas fraction but decrease with increasing ϵ. The hatched regions are the same as in Fig. 4.