Average C¹⁸O (J=2–1) spectrum for 15 different protoclusters. Two vertical green dashed lines indicate the velocity cut-off to measure the systemic velocity (V_sys). The red dashed line represents the measured V_sys based on the C¹⁸O (J=2–1) line. In the W51-E and W51-IRS2 regions, the black dashed line indicates the minima between the separation of the two broad clouds. Symbols Y, I, towards and E in the figures denote young, intermediate and evolved protoclusters respectively (see Table 1).