The TNG50-SKIRT Atlas: Multi-wavelength properties of galaxy discs in the TNG50 simulation

We investigate the wavelength dependence of disc scale lengths in galaxies from the TNG50-SKIRT Atlas (TSA), and assess whether the simulated galaxies reproduce the structural properties and scaling relations observed in real galaxies. We measure disc scale lengths from 1D radial surface-brightness profiles of multi-wavelength synthetic images. The disc-dominated region is defined interactively per galaxy and applied consistently across bands. We restrict the analysis to nearly face-on, star-forming galaxies and retain only systems whose r-band profiles are well described by an exponential model. The resulting scale lengths are compared to observational samples and analysed using both dust-attenuated and dust-free images, as well as intrinsic stellar population maps. We find that the majority of TSA galaxies exhibit exponential discs in realistic mock observations. TSA galaxies reproduce the observed disc scale length-stellar mass (luminosity) relation, but are systematically larger at fixed mass or luminosity. The dependence of disc scale length on morphology is weak. Disc scale lengths correlate strongly across wavelengths, but decrease systematically with increasing wavelength. When normalised by the stellar-mass scale length, TSA galaxies exhibit a monotonic trend consistent with observations over the optical-to-NIR range. Comparing dust-free and dust-attenuated images and linking the results to intrinsic galaxy properties, we find that stellar population gradients are the primary driver of this wavelength dependence. In particular, radial gradients in stellar age dominate the effect, while metallicity gradients play a secondary role. Dust attenuation further enhances the wavelength dependence at short wavelengths but is not its main origin. Residual offsets in absolute scale length indicate the remaining differences between simulated and observed galaxy structure.