2025MNRAS.544.4403G

We use the TNG50 cosmological simulation and three-dimensional radiative transfer post-processing to generate dust-aware synthetic observations of galaxies at $3 \leqslant z \leqslant 6$ and $\log _{10} \left(M_\ast / \rm{M}_{\odot }\right) \geqslant 8.5$, tailored to match the depth and resolution of current deep JWST surveys (NGDEEP and JADES). We analyse the performance of spectral energy distribution (SED) fitting on the simulated sample, focusing on the recovery of photometric redshift and stellar mass. At $z \leqslant 5$, we find that $>$90 per cent of redshifts are recovered within $\pm$0.2, but performance declines at $z = 6$. Stellar masses are generally well-recovered within a factor of 2, but are systematically underestimated regardless of redshift, a trend that is more pronounced at the high-mass end $\left(\log _{10}\left(M_\ast / \rm{M}_{\odot }\right) \geqslant 10\right)$. In addition, we study the observer-frame colours of galaxies in this redshift range as well as the SED-inferred UVJ diagram. We find that TNG50 galaxies broadly follow the tendencies marked by observations, but tend to be slightly redder at lower masses and bluer at higher masses, regardless of redshift. Finally, using a colour-based definition of quiescence, we determine the fraction of quiescent galaxies as a function of stellar mass at $3 \leqslant z \leqslant 6$, which we find to be broadly consistent with observations.