Face-on gas density map of a snapshot of our high-z disk model, used to analyze the contribution of “giant clumps” to the global CO SLED. The gray-scale inset displays a mock HST-like optical observation (B-band rest-frame emission with a Gaussian PSF of FWHM 500 pc), which shows an irregular structure with a few giant clumps and some spiral arms, typical of real z ≈ 2 star-forming galaxies (Elmegreen et al. 2007). The seven giant clumps (with stellar masses above 10⁸ M_⊙) are circled on the gas map, with the clump radius computed as in Bournaud et al. (2014). These seven clumps contain about half of the total SFR and one third of the total gas mass. The SLEDs corresponding to the emission for the clump regions and from the rest of the galaxy are analyzed in Fig. 4. Gas number densities are in cm^-3, and the map is smoothed at a 10 pc scale.