a) Integrated intensity map of C¹⁸O 1–0 emission detected with the SEST by Haikala et al. (2005) in scale, smoothed to an effective angular resolution of 3′ (filled circle). The extinction map of Fig. 1 is overlaid as contours that increase from 3 to 18 mag by step of 3 mag. b) Correlation between the C¹⁸O 1–0 integrated intensity and the visual extinction. Only data points with a C¹⁸O 1–0 integrated intensity higher than 0.1 K km s^-1 (>3σ for 90% of the map) and A_V > 2 mag are plotted. The dotted line marks the extinction A_V = 6 mag above which a linear fit with a fixed slope of 1 (in log-log space) was performed. The red line shows the result of this fit. Note that all visual extinction values where derived assuming the standard extinction law and have to be multiplied by 0.85 for A_V above  ~6 mag (see Appendix B.3).