[CII] flux density in mJy as a function of frequency (GHz) for the galaxies in our sample. Black lines are the total [CII] emission arising from the full system (galaxy + diffuse [CII] halo) and green solid lines are the sum of the emissions coming from each component of the merging system when we consider the FWHM_x × FWHM_y aperture. Green hatched areas show the emission coming from the diffuse [CII] halo and the [CII] fraction is in green boxes (see Sect. 3 for the definition of this quantity and discussion).