HI within and around observed and simulated galaxy discs

Extragalactic gas accretion and outflows driven by stellar and active galactic nucleus (AGN) feedback are expected to influence the distribution and kinematics of gas in and around galaxies. Atomic hydrogen (H I ) is an ideal tracer of these processes, and it is uniquely observable in nearby galaxies. Here we made use of wide-field (1° ×1°), spatially resolved (down to 22″), high-sensitivity (∼10 18 cm −2 ) H I observations of five nearby spiral galaxies with stellar mass of ∼5 × 10 10 M ⊙ , taken with the MeerKAT radio telescope. Four of these were observed as part of the MHONGOOSE survey. We characterise the main H I properties in regions of a few hundred kiloparsecs around the discs of these galaxies, and compare them with synthetic H I data from a sample of 25 similarly massive star-forming galaxies from the TNG50 (20) and FIRE-2 (5) suites of cosmological hydrodynamical simulations. Overall, the simulated systems have H I and molecular hydrogen (H 2 ) masses in good agreement with the observations, but only when a pressure-based H 2 recipe is employed. The other recipes that we tested overestimate the H 2 -to-H I mass fraction by up to an order of magnitude. On a local scale, we find two main discrepancies between the observed and simulated data. First, the simulated galaxies show a more irregular H I morphology than the observed galaxies, due to the presence of H I with column density < 10 20 cm −2 up to ∼100 kpc from the galaxy centre, even though they inhabit more isolated environments than the observed targets. Second, the simulated galaxies and in particular those from the FIRE-2 suite, feature more complex and overall broader H I line profiles than the observed galaxies. We interpret this as being due to the combined effect of stellar feedback and gas accretion, which lead to a large-scale gas circulation that is more vigorous than in the observed galaxies. Our results indicate that, with respect to the simulations, gentler processes of gas inflows and outflows are at work in the nearby Universe, leading to more regular and less turbulent H I discs.