M17 SW mosaics of the seven positions observed by upGREAT. (a) Comparison between C¹⁸O J = 1–0 and the [¹²C II] Gaussian components from the double layer model. Scaled-up C¹⁸O observationsare in black, [¹²C II] background components are in blue and [¹²C II] foreground optical depth components are in pink. (b) Comparison between C¹⁸O J = 1–0 and the [¹²C II] Gaussian components from the single layer model. Scaled-up C¹⁸O observationsare in black, [¹²C II] high density low temperature components are in green ([C II] high d.) and [¹²C II] low density high temperature components are in red ([C II] low d.).