Plot of the average dust surface density against the deprojected galactocentric distances for 134 of our GDCs with reliable mass measurements. The coloured symbols all indicate GDCs that are associated with D88 HII regions. The GDCs that also coincide with regions that were observed by APEX (see Sect. 4.7) are indicated by the blue filled circles (CO detections), yellow unfilled squares (marginal CO detections), and red crosses (no CO detections). The error bars denote the uncertainty in the dust surface density estimates. The dotted and dashed horizontal lines indicate constant dust surface density values of 0.05 and 0.1 M_⊙ pc⁻², respectively.At least 87% of the sample have dust surface densities sufficient for the formation of molecular gas (see the text).