Why simulations keep missing faint early galaxies

Forward-modeling TNG100 and EAGLE simulations into synthetic CANDELS images reveals a persistent deficit of faint galaxies at redshifts above three across all observed fields. Crucially, even completeness-corrected observed counts exceed the raw simulation catalogs, ruling out the explanation that the missing galaxies are simply too faint to detect. The simulations favor extended, low-surface-brightness systems, while real high-redshift galaxies tend to be compact with bright cores—a structural mismatch that no tuning of image depth can fix. The shortfall implicates gaps in how current simulations handle early star formation, feedback, and dust.