Relative [C ii] emission as a function of the 158 μm continuum luminosity for W31C, W49N, and W51. For each source, the black triangles show the spectrally unresolved PACS data, the blue squares the contribution of the foreground absorption measured by HIFI, the red squares the intrinsic source emission measured by HIFI, and the green stars the total emission and absorption measured by HIFI. The velocity intervals for the integration are given in each plot. In all plots the points on the left side correspond to the map edges while those on the right side correspond to the massive star forming regions associated with the dust continuum peak.