Variations of ϵ_ν for the THEMIS model under various heating conditions. Left panel: ϵ_ν(250 μm) versus I_ν(250 μm)/I_ν(500 μm) for radiation fields scaled on the LISRF, with different values for γ, the fraction of dust heated by a radiation field distribution (α = 2 and U_max = 10⁷ for γ ≠ 0; Eq. B.1); and for ISRFs dominated by UV (young stars) or red (old stars) radiation (γ = 0; see text for details). Contour plots (gray lines) and averages (black line) from the analysis in the main text are shown for reference (Fig. 3). Right panel: ϵ_ν versus λ for the central values of the I_ν(250 μm)/I_ν(500 μm) bins and the same cases as in the left panel. Dots represent the emissivities integrated over the Herschel filter response functions (for the LISRF, γ = 0 case only).