Panel a) The relation between the surface density of the molecular gas and the SFR per area for morphological types T = 3–5, calculated by dividing the molecular gas mass and the SFR derived from L_FIR by the surface of the galaxies, π(D₂₅/2)². The full line shows the bisector best fit for this sample, and the dashed line the best fit to the sample of all morphological types. Panel b) The same relation with the total (atomic + molecular) gas surface density. The blue triangles show the galaxies with either an upper limit in HI or in CO. The gas surface density for these galaxies has a lower and an upper limit which for the galaxies in this figure are very close together (within the size of the triangle).