a) (Upper panel) Histograms showing the number density of galaxies (number of galaxies per unit interval, as shown) as a function of log(t_dep) for normal galaxies (L_IR < 10¹¹   L_⊙, gray filled histogram) and luminous infrared galaxies (L_IR > 10¹¹   L_⊙, empty histogram). b) (Middle panel) Same as a) but representing the number density of galaxies as a function of log (t_dyn). c) (Lower panel) Same as a) but representing the number density of galaxies as a function of log (t_dep/t_dyn).