Left: fraction of M_dust heated by radiation fields stronger than that of the diffuse ISM (or equivalently by U > U_min) as a function of the L₂₄/L_IR ratio for star-formation-dominated sources with EW_6.2 > 0.5  μm (black circles) and 0.2  μm <EW_6.2 ≤ 0.5  μm (purple circles). The orange line depicts the best fit to the data. Right: dust mass heated by U > U_min versus . Both quantities are normalised by L_IR. The purple lines donate the best fit to the data and a scatter of 0.2 dex.