NGC 4254: top left: star formation rate (from UV+TIR) in contours on a greyscale map of the total gas surface density. Contour levels are (1,2,4,8,16,32,64,128)  3 × 10^-4 M_⊙ kpc^-2 pc^-1. Greyscale levels are (1,2,4,8,16,32,64,128) M_⊙ pc^-2. Top right: total gas surface density contours on the molecular gas fraction Σ_H2/Σ_HI. Greyscale levels are (1,2,4,8,16,32,64) × 0.1. Middle left: total gas surface density contours on the star formation timescale . Greyscale levels are (1,10,20,30,40,50,60,70,80,90,100) 10 Myr. Middle right: star formation rate  as a function of the molecular gas surface density Σ_H2. The vertical set of points at the lowest Σ_mol are upper limits for Σ_mol. Lower left: star formation rate as a function of the atomic gas surface density Σ_HI. Lower right: star formation rate as a function of the total gas surface density Σ_g = Σ_H2 + Σ_HI. The solid line corresponds to a star formation timescale of t_∗ = 2 Gyr. The arrow takes into account the CO detection limit.