a) Distribution of extinction across the map. The area for A_V < 3 regions is limited by the map boundary. b) Mean surface density of starless (+ Class 0) cores as a function of gas column density as measured by A_V. For each A_V value, the probability distribution for the true value of the mean surface density is displayed in greyscale. The corresponding colorbar on the right is labeled in natural logarithm. The insert is a zoom on the low-extinction range. The best-fit power-law curve to the mean surface density is shown in green/black (see Sect. 6.1.1 for details).