Specific star formation rate (sSFR, in Gyr^-1) as a function of the stellar mass surface density (Σ_⋆, in M_⊙ pc^-2) as predicted by a simple relation where the hydrostatic pressure and the thrust of the mechanical energy generated by intense star formation are equal. To span a range of possible sSFR in such a scenario, we vary the input values of the gas fraction, f_g, or the velocity dispersion ratio of the gas and the stars, σ_gas/σ_stars. The red solid lines represent this equality 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 this equality for a constant gas fraction, f_g = 0.3, and a range of σ_gas/σ_stars = 0.25−1.25 in steps of 0.25. The hatched regions indicate approximately the observed range of stellar mass surface densities and sSFR at z ~ 2 (diagonally hatched region) and z ~ 5−7 (cross-hatched region; see text for details).