Panel a: [CII] spectra (yellow) of the individual galaxy components in the merging system are shown: #A in the upper panel, and the combined spectrum of #B, #C, #D in the lower panel. Solid coloured lines represent Gaussian models of the spectra. Panel b: velocity-integrated [CII] flux map obtained integrating the broad velocity range [ − 500 : +500] km s⁻¹, to visualise the total [CII] emission arising from the full system. Black plus symbols show the rest-frame UV centroids of #A and #B. The dashed blue ellipses defined in Fig. 3 (see Sect. 3.3) are shown for reference. Black solid (dashed) contours indicate the positive (negative) significance levels at [2, 4, 6]σ of [CII] flux, where σ_[CII] = 0.13 Jy km s⁻¹ beam⁻¹. Green contours indicate (as in Fig. 3) the FIR-continuum significance levels [3, 4]σ, where σ_cont = 59 μJy beam⁻¹. The grey solid line indicates the 1σ level of the total [CII] emission, and it is used as a reference aperture for the extraction of the full [CII] spectrum arising from the entire system. The ALMA beam size is given in the bottom-left corner. A scale of 15 kpc is shown on the right side. North is up and east is to the left.