High-resolution, 3D radiative transfer modelling IV. AGN-powered dust
  heating in NGC 1068
  Dust emission, an important diagnostic of star formation and ISM mass
throughout the Universe, can be powered by sources unrelated to ongoing star
formation. In the framework of the DustPedia project we have set out to
disentangle the radiation of the ongoing star formation from that of the older
stellar populations. This is done through detailed, 3D radiative transfer
simulations of face-on spiral galaxies. In this particular study, we focus on
NGC 1068, which contains an active galactic nucleus (AGN). The effect of
diffuse dust heating by AGN (beyond the torus) was so far only investigated for
quasars. This additional dust heating source further contaminates the broadband
fluxes on which classic galaxy modelling tools rely to derive physical
properties. We aim to fit a realistic model to the observations of NGC 1068 and
quantify the contribution of the several dust heating sources. Our model is
able to reproduce the global spectral energy distribution of the galaxy. It
matches the resolved optical and infrared images fairly well, but deviates in
the UV and the submm. We find a strong wavelength dependency of AGN
contamination to the broadband fluxes. It peaks in the MIR, drops in the FIR,
but rises again at submm wavelengths. We quantify the contribution of the dust
heating sources in each 3D dust cell and find a median value of 83% for the
star formation component. The AGN contribution is measurable at the percentage
level in the disc, but quickly increases in the inner few 100 pc, peaking above
90%. This is the first time the phenomenon of an AGN heating the diffuse dust
beyond its torus is quantified in a nearby star-forming galaxy. NGC 1068 only
contains a weak AGN, meaning this effect can be stronger in galaxies with a
more luminous AGN. This could significantly impact the derived star formation
rates and ISM masses for such systems.
