Spitzer 8 μm image of the molecular complex G351. The white contour is 5% of the C¹⁸O (2–1) peak intensity (17.6 K km s⁻¹) integrated inthe velocity range − 3.5 ± 0.5 km s⁻¹. The green triangles denote the ATLASGAL clumps identified in Paper I (see Table 1, labelled as C1, C2...). The skeleton of G351 and that of the branches is shown by the white dashed line. The white dotted lines indicate the region observed in CO isotopologues with APEX. The B1–B8 labels indicate the eight branches identified in the column density map (see Sect. 4.1). The white square on the south-west of G351 indicates the position of MSX IRDC G351.64–00.46 (Simon et al. 2006).