Emissivity at 250 μm, ϵ_ν(250 μm) versus the I_ν(250 μm)/I_ν(500 μm) ratio (assuming ). In the left panel, individual pixels are shown; in the right panel, gray contours encompass 95, 75, 50, and 25% of the sample. The mean ϵ_ν(250 μm) obtained after averaging pixels over five bins in the range of 3 ≤ I_ν(250 μm)/I_ν(500 μm)< 8 is also shown: in the left panel, for each galaxy separately; in the right panel, for the total sample (and the total sample but NGC 5457). The left panel further shows the average errorbars for the central I_ν(250 μm)/I_ν(500 μm) bin; the right panel, the expectation for the “exact” (red contours) and “null” (blue binned averages) hypotheses.