Toppanel: A_V extinction map derived by Dobashi et al. (2005), with the Herschel low column density map contours overlaid in white. The contours start from 3σ, with each consecutive contour being 1.5 times the previous level. Lower panel: A_V against the column density along the line of sight. The grey dashed line is the fit N(H₂)∕A_V =  1 × 10²¹ cm⁻² mag⁻¹.The orange points are those that were excluded from the fitting process, as they are in very high extinction areas, where the optical extinction relation breaks down.