Whole sample stacked integrated spectrum. First panel from top: number of sources contributing to the stack at different velocities. Second panel from top: [CII] flux density as a function of velocity, in spectral bins of 60 km s⁻¹. The red curve represents the best-fit 2 Gaussian components model: the combination of a core component (blue) and a broad component (green) is needed to properly reproduce the data. Labels indicate the number of stacked sources and the luminosity of the broad [CII] wings. The inset shows a zoom on the broad component. Third panel from top: residuals from the subtraction of the core component (blue line in the second panel). The green curve shows the best fit broad component. Fourth panel: residuals from the two Gaussian components fitting. The 1σ rms of the spectrum is also indicated by the shaded region.