a) On logarithmic scale, H₂ column density map of RCW 120 at 36 6 resolution with 870 μm emission from LABOCA (in yellow countours), the final sample of sources (black dots) and the three prestellar clumps (red dots). Condensations observed at 870 μm are identified following the labelling in DEH09. The density values range from 7 × 10²¹ cm^-2 to 4 × 10²³ cm^-2. b) High resolution H₂ column density map of RCW 120 at 18 2 resolution (in red) and Hα emission (in blue) from the SuperCOSMOS Hα Survey. The column density values range from 7 × 10²¹ cm^-2 to 9.4 × 10²³ cm^-2.