Left: line integrated ¹³CO 1→0 emission in colour scale between v = 72 and 84 km s^-1, the velocity range of the bulk emission of the IRDC, and the associated GMC. The white contour indicates the 3.5 K kms^-1 level, corresponding to A_v = 2, which defines the extent of the GMC and was used for mass estimation. The Herschel column density is overlaid as black contours (levels 2 to 6 × 10²² cm^-2 in steps of 10²² cm^-2) and the IRDC is outlined by an ellipse (Simon et al. 2006b). Right: H₂-column density map of the centre region of the GMC, obtained from ¹³CO 1→0. The white-dashed contour indicates a column density of 1.25 × 10²² cm^-2, the remaining black contours the levels 1.5, 1.75, and 2 × 10²² cm^-2.