2026ApJ...998...56L

With new atomic-hydrogen (H I) observations of FAST Extended Atlas of Selected Targets Survey (FEASTS), we present the first statistical comparison of H I morphology between observations and cosmological simulations, focusing on low-column-density (∼10¹⁸ cm^–2) regions of Milky Way-like central galaxies. We select a 330-galaxy sample from IllustrisTNG50 (TNG50) matched to 33 FEASTS galaxies by stellar and H I masses, and mock observe them at the FAST resolution and depth at corresponding inclinations and distances for a fair comparison. In contrast to FEASTS, abnormally irregular and extended morphology is found in more than one-third of TNG50 galaxies, especially those that are massive and H I poor. Stellar feedback is the property that most significantly correlates with the H I morphological deviation from observations, although these deviations mostly occur at a high stellar or black-hole mass. These results indicate that, in TNG50, stellar feedback significantly influences the H I morphology at ∼10¹⁸ cm^–2, while active galactic nucleus (AGN) feedback has not so direct a role as expected. With an additional sample from Auriga, we find that the magnetic field may help H I to be more regular in its morphology, while improving the mass resolution does not alleviate the discrepancy from observation. This study reveals the potential of constraining future simulations of galaxies by observing low-column-density H I.