a: variance-weighted stacked [C II] spectrum of all galaxies in our sample is shown, in velocity bins of 25 km s⁻¹. The orange (blue) line shows the single-Gaussian (two-Gaussian) best-fit. The red and the green line represent the narrow and broad components of the two-Gaussian model, respectively. A zoom of the velocity range [ − 600 :   + 600] km s⁻¹ is shown in the inset. A histogram containing the number of objects per channel contributing to the stacking is shown in the top panel. b: residuals from the single-Gaussian (two-Gaussian) best-fit are shown in red (blue), in velocity bins of 50 km s⁻¹.