Emissivity ϵ(250 μm) as a function of the F(250 μm)/F(500 μm) ratio. When data on the abscissa (and/or ordinate) are estimated at a level below 2σ, the x (and/or y) error is omitted and an open symbol is used. The red error bar shows the MW estimate from HeVICS. The mean of ϵ(250 μm) and its standard deviation for five bins in F(250 μm)/F(500 μm) is shown by the black error bars (connected by a solid line). The F(250 μm)/F(500 μm) range and the number of galaxies is given for each bin.