Correlation between FIR and CO luminosities, for our sample (full black circles, and arrows for upper limits) and the other points from the literature (same symbols as in Fig. 6). The 3 lines are for L_FIR/M(H₂) = 10, 100 and 1000 L_⊙/M_⊙  from bottom to top, assuming a conversion factor α = 0.8 M_⊙ (K  km s^-1 pc²)^-1. The three lines correspond to gas depletion time-scales of 580, 58 and 5.8 Myr respectively.