The histogram of the luminosity densities of M 33 produced by the “cool” dust component (green) and the “warm” dust component (red and black). The “cool” part of the luminosity was derived by fitting the SED with a two-component modified blackbody (β = 1.5) model. The “warm” part of the luminosity is computed using the residual fluxes of the SED, derived after subtracting the “cool” dust component (see Fig. 5), and by adopting two approximations consisting of integrating the residual SED by a constant interpolation (black line) and a linear interpolation (red line) between the successive fluxes (see the text for more details).