2024ApJ...969...28C

The presence of dense, neutral hydrogen clouds in the hot, diffuse intragroup and intracluster (IC) medium is an important clue to the physical processes controlling the survival of cold gas and sheds light on cosmological baryon flows in massive halos. Advances in numerical modeling and observational surveys mean that theory and observational comparisons are now possible. In this paper, we use the high-resolution TNG50 cosmological simulation to study the H I distribution in seven halos with masses similar to the Fornax galaxy cluster. Adopting observational sensitivities similar to the MeerKAT Fornax Survey (MFS), an ongoing H I survey that will probe to column densities of 10¹⁸ cm^–2, we find that Fornax-like TNG50 halos have an extended distribution of neutral hydrogen clouds. Within 1 R_vir, we predict the MFS will observe a total H I covering fraction of ∼12% (mean value) for 10 kpc pixels and 6% for 2 kpc pixels. If we restrict this to gas more than 10 half-mass radii from galaxies, the mean values only decrease mildly, to 10% (4%) for 10 (2) kpc pixels (albeit with significant halo-to-halo spread). Although there are large amounts of H I outside of galaxies, the gas seems to be associated with satellites, judging both by the visual inspection of projections and by comparison of the line of sight velocities of galaxies and IC H I.