The distribution of atomic hydrogen in EAGLE galaxies: morphologies, profiles, and HI holes

We compare the mass and internal distribution of atomic hydrogen (H I) in 2200 present-day central galaxies with M_star > 1010 M_⊙ from the 100 Mpc EAGLE 'Reference' simulation to observational data. Atomic hydrogen fractions are corrected for self-shielding using a fitting formula from radiative transfer simulations and for the presence of molecular hydrogen using an empirical or a theoretical prescription from the literature. The resulting neutral hydrogen fractions, M_HI+H2/M_star, agree with observations to better than 0.1 dex for galaxies with M_star between 10¹⁰ and 10¹¹ M_⊙. Our fiducial, empirical H2 model based on gas pressure results in galactic HI mass fractions, M_HI/M_star, that agree with observations from the GASS survey to better than 0.3 dex, but the alternative theoretical H₂ formula from high-resolution simulations leads to a negative offset in M_HI/M_star of up to 0.5 dex. Visual inspection of mock HI images reveals that most HI discs in simulated HI-rich galaxies are vertically disturbed, plausibly due to recent accretion events. Many galaxies (up to 80 per cent) contain spuriously large HI holes, which are likely formed as a consequence of the feedback implementation in EAGLE. The HI mass-size relation of all simulated galaxies is close to (but 16 per cent steeper than) observed, and when only galaxies without large holes in the HI disc are considered, the agreement becomes excellent (better than 0.1 dex). The presence of large HI holes also makes the radial HI surface density profiles somewhat too low in the centre, at σ_HI > 1M_⊙ pc^-2 (by a factor of ≲2 compared to data from the Bluedisk survey). In the outer region (σ_HI < 1M_⊙ pc^-2), the simulated profiles agree quantitatively with observations. Scaled by HI size, the simulated profiles of HI-rich (M_HI > 10^9.8M_⊙) and control galaxies (10^9.1 M_⊙ >M_HI > 10^9.8 M_⊙) follow each other closely, as observed.