4.3. The origin of dust heating
There has been a long standing question in the literature of what is the contribution of different stellar populations to the heating of dust. This question is equivalent to the question of the relative attenuations (averaged over viewing angle) of individual stellar populations, for example in the bulge and in the old and young stellar disks. The self-consistent models of optical/FIR/submm SEDs provide direct means to address this question.
In the modelling of the face-on spiral NGC 6946, Bianchi et al.  found that it is the old stellar population that predominantly powers the dust emission, with only 40% of the emission powered by UV photons. However this result assumed that more than 50% of the absorbed UV radiation is channeled directly into the MIR, whereas self-consistent calculations of this percentage show that most of the UV luminosity is actually re-emitted in the FIR. For the same galaxy both Silva et al.  and Sauty et al.  found that most of the dust heating comes from the young stellar population. In the modelling of the edge-on galaxy NGC 891 Popescu et al.  also found that it is the young stellar population that predominantly heats the dust, powering about 70% of the total dust luminosity. Furthermore, because the dust emission from the different components has different characteristic temperatures, one can determine the fractional contribution of the different stellar populations as a function of infrared wavelength. For the case of NGC 891 (see Fig. 12) one can see that, as expected, HII regions dominate the dust emission at shorter wavelengths. The really interesting feature of this diagram is the predominance of the UV heating in the submm regime. As explained by Tuffs & Popescu , this can be understood as follows: the coldest grains are those which are in weaker radiation fields, either in the outer optically thin regions of the disk, or because they are shielded from radiation by optical depth effects. In the first situation the absorption probabilities of photons are controlled by the optical properties of the grains, so the UV photons will dominate the heating. The second situation arises for dust associated with the young stellar population, where the UV emissivity far exceeds the optical emissivity.
Figure 12. The fractional contribution of different stellar components to the dust emission, as a function of FIR/submm wavelength, taken from Popescu et al. .