Pixel-to-pixel correlations between various tracers of the physical conditions and the gas-to-dust mass ratio. Results are shown for the two models, at spatial resolution R4. The color scale represents the density of pixels in various bins of the two parameters. The overplotted grey error bars are the trends binned over the x-axis parameter. The bin size is defined so that the same number of pixels falls within each bin. That is the reason why the bins are very narrow in the center, and there are only a few points in the outer parts where the pixel density is low. The central value is the median gas-to-dust mass ratio, and the error bars account for the pixel-to-pixel scatter, which is similar to or larger than the typical intrinsic error bar of a single pixel. The yellow and purple dashed error bars, with the star symbol, represent the 90% confidence uncertainities on and , respectively (Table 5). This complex figure is explained in details in Sect. 4.3.5.