Pixel-to-pixel distribution of the gas-to-dust mass ratio for R4 (54 pc). The two histograms of the top panel are the pixel probability distribution of the gas-to-dust mass ratio for the two models. The spread of the distribution reflects the pixel to pixel spread fluctuations of the ratio. The actual error on the total G_dust is smaller than this spread (Table 7). For comparison, we show the values of (Eq. (20)), (Eq. (15)), and (Eq. (22)). In the two lower panels, each histogram is repeated and is decomposed according to its dust mass surface density. The hatched components correspond to lower surface densities (below the arbitrary limit indicated in each panel). The yellow and purple error bars, with the star symbol, represent the uncertainities on and , respectively (Table 5; solid line: 50%; dashed line: 90%).