The Structural Abundance Crisis of Massive Galaxies in Current Cosmological Simulations

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Abstract The internal structure of galaxies encodes the complex baryon cycle driven by gas accretion, star formation, and feedback, together with secular evolution and environmentally driven processes such as mergers and tidal interactions. We present a population-level census of massive nearby galaxies ( log ( M ⋆ / M ⊙ ) > 10 ) by comparing Hyper Suprime-Cam Subaru Strategic Program observations with matched mock images from IllustrisTNG, EAGLE, and SIMBA. Using a consistent, like-for-like imaging pipeline, we construct structural abundance functions (SAFs) for key morphological parameters, revealing a structural abundance crisis. Across Sérsic index, concentration, size, and ellipticity, all simulations exhibit large, systematic discrepancies (>5 σ ; RMSE ∼ 0.2–1.8 dex), typically corresponding to abundance differences of factors of several. While individual simulations display diverse failures—underproducing or overproducing compact spheroids, extended or round galaxies—all underproduce highly flattened disks. Although TNG shows the closest agreement and SIMBA the largest offsets, this shared failure indicates that current models—despite matching global demographics such as the stellar mass function—do not uniquely constrain internal galaxy structure. Our results demonstrate that agreement in integrated observables can mask fundamental shortcomings in the modelling of mass and angular momentum redistribution. We therefore establish SAFs as a stringent, multidimensional, and observationally accessible benchmark for testing and calibrating next-generation galaxy formation models in the era of upcoming deep, wide-field surveys.

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