Distributions of the K_s-band (turquoise histogram) and [CII] (hatched histogram) flux ratios between the components of each merging system. Inset: comparison between the [CII] and K_s-band ratios for the sources having this information in common, both at z ∼ 4.5 and z ∼ 5.5 (represented as yellow and gray circles, respectively). The green area marks the region of the figure with μ_K <  4. The dashed black line reports the 1:1 relation between the two quantities.