The dust-continuum size of TNG50 galaxies at z = 1–5: a comparison with the distribution of stellar light, stars, dust, and H2

We present predictions for the extent of the dust-continuum emission of main-sequence galaxies drawn from the TNG50 simulation in the range z = 1-5. We couple the radiative transfer code SKIRT to the output of the TNG50 simulation and measure the dust-continuum half-light radius of the modelled galaxies, assuming a Milky Way dust type and a metallicity-dependent dust-to-metal ratio. The dust-continuum half-light radius at observed-frame 850 μm is up to ~75 per cent larger than the stellar half-mass radius, but significantly more compact than the observed-frame 1.6 μm (roughly corresponding to H band) half-light radius, particularly towards high redshifts: the compactness compared to the 1.6 μm emission increases with redshift. This is driven by obscuration of stellar light from the galaxy centres, which increases the apparent extent of 1.6 μm disc sizes relative to that at 850 μm. The difference in relative extents increases with redshift because the observed-frame 1.6 μm emission stems from ever shorter wavelength stellar emission. These results suggest that the compact dust-continuum emission observed in z > 1 galaxies is not (necessarily) evidence of the build-up of a dense central stellar component. We find that the dust-continuum half-light radius closely follows the radius containing half the star formation and half the dust mass in galaxies and is ~80 per cent of the radius containing half the H2 mass. The presented results are a common feature of main-sequence galaxies.