Mean values, per Hubble stage bin, of the fraction of the luminosity of the old and the young stellar components (left and right panels, respectively) used for the dust heating. In each panel the mean value of the ratio of the luminosity absorbed by the dust to the unattenuated luminosity of the specific stellar component (, ) is shown as crossed bars, while the mean value of the ratio of the attenuated luminosity of the specific stellar component to the unattenuated luminosity (, ) is shown as solid bars. The mean values are provided in Table 4.