The mass-dependent UVJ diagram at cosmic noon

Context. The UVJ color-color diagram is a widely used diagnostic to separate star-forming and quiescent galaxies. Observational data from photometric surveys reveal a strong stellar mass trend, with higher-mass star-forming galaxies being systematically more dust-reddened. Aims. We analyze the UVJ diagram in the TNG100 cosmological simulation at cosmic noon (z ≈ 2). Specifically, we focus on the trend between UVJ colors and mass, which has not been reproduced in any cosmological simulation thus far. Methods. We applied the SKIRT dust radiative transfer code to the TNG100 simulation to generate rest-frame UVJ fluxes. These UVJ colors were then compared to observational data from several well-studied extragalactic fields from the CANDELS/3D-HST programs, augmented by recent JWST/NIRCam photometry. Results. Quiescent and low-mass (M⋆ ≲ 1010.5 M⊙) galaxies at cosmic noon do not require significant levels of dust reddening, as opposed to massive (M⋆ ≳ 1011 M⊙) star-forming galaxies. An extensive range of possible dust models fall short of the required dust reddening in V − J color for massive star-forming galaxies, with the simulated galaxies being too blue by ≈0.9 mag. Conclusions. We find that only variations in the star-to-dust geometries of the simulated galaxies are able to yield V − J colors that are red enough to match the observations. A toy model with isolated dust screens around younger stellar populations (with ages below ∼1 Gyr) can reproduce the observational data, while all “conventional” dust radiative transfer models (where the dust distribution follows the metals in the interstellar medium) fail to achieve the required V − J colors.