The ¹³CO(2−1) and C¹⁸O(2−1) spectra extracted towards SMM 3 (upper panel) and the position indicated in Fig. 4 (lower panel). Hyperfine-structure fits to the ¹³CO lines, and single Gaussian fits to the C¹⁸O lines are overlaid in green. The red dashed line overlaid on the spectra towards SMM 3 indicates the systemic velocity of SMM 3 as derived from C¹⁷O(2−1) in Paper III. In the lower panel, the red dashed line shows the radial velocity of the C¹⁸O(2−1) line.