Gas fraction. Shown is f_gas​ = ​M_g/(M_g​ + ​M_s) within R ≃ 2R_e as a function of redshift at the FFB threshold, based on Eq. (38), for ϵ_max​ = ​0.5 in the shell and disk models. The shaded bands indicate the uncertainty due to ϵ_max ranging from 0.2 (top) to 1 (bottom). The gas fraction is rather low in the whole FFB regime, largely due to the low mass-loading factor associated with the high SFE.