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We compare the mass and internal distribution of atomic hydrogen (H i) in 2200 present-day central galaxies with Mstar > 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_ H\, I+H₂ / Mₛₜₐᵣ|, agree with observations to better than 0.1 dex for galaxies with Mstar between 1010 and 1011 M⊙. Our fiducial, empirical H2 model based on gas pressure results in galactic H i mass fractions, |M_ H\, i/ Mₛₜₐᵣ|, 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_ H\, i/ Mₛₜₐᵣ| of up to 0.5 dex. Visual inspection of mock H i images reveals that most H i discs in simulated H i-rich galaxies are vertically disturbed, plausibly due to recent accretion events. Many galaxies (up to 80 per cent) contain spuriously large H i holes, which are likely formed as a consequence of the feedback implementation in EAGLE. The H i 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 H i disc are considered, the agreement becomes excellent (better than 0.1 dex). The presence of large H i holes also makes the radial H i surface density profiles somewhat too low in the centre, at |Σ _ H\, i > 1 \,M\, pc⁻²| (by a factor of ≲ 2 compared to data from the Bluedisk survey). In the outer region (|Σ _ H\, i < 1 \,M\, pc⁻²|), the simulated profiles agree quantitatively with observations. Scaled by H i size, the simulated profiles of H i-rich (|M_ H\, i> 109.8 \,M|) and control galaxies (|109.1\, \,M> M_ H\, i> 109.8\, \,M|) follow each other closely, as observed.
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Bahé et al. (2015) studied this question.
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