The distribution of neutral hydrogen around high-redshift galaxies and quasars in the EAGLE simulation

The observed high covering fractions of neutral hydrogen ( ${\rm H\,\small {\rm I}}$ ) with column densities above ∼1017 cm−2 around Lyman-Break Galaxies (LBGs) and bright quasars at redshifts z ∼ 2–3 has been identified as a challenge for simulations of galaxy formation. We use the Evolution and Assembly of Galaxies and their Environment (EAGLE) cosmological, hydrodynamical simulation, which has been shown to reproduce a wide range of galaxy properties and for which the subgrid feedback was calibrated without considering gas properties, to study the distribution of ${\rm H\,\small {\rm I}}$ around high-redshift galaxies. We predict the covering fractions of strong ${\rm H\,\small {\rm I}}$ absorbers ( ${N_{\rm H\,\small {I}}}\gtrsim 10^{17} \,{\rm cm^{-2}}$ ) inside haloes to increase rapidly with redshift but to depend only weakly on halo mass. For massive (M 200 ≳ 1012M⊙) haloes, the covering fraction profiles are nearly scale-invariant and we provide fitting functions that reproduce the simulation results. While efficient feedback is required to increase the ${\rm H\,\small {\rm I}}$ covering fractions to the high observed values, the distribution of strong absorbers in and around haloes of a fixed mass is insensitive to factor of 2 variations in the strength of the stellar feedback. In contrast, at fixed stellar mass the predicted ${\rm H\,\small {\rm I}}$ distribution is highly sensitive to the feedback efficiency. The fiducial EAGLE simulation reproduces both the observed global column density distribution function of ${\rm H\,\small {\rm I}}$ and the observed radial covering fraction profiles of strong ${\rm H\,\small {\rm I}}$ absorbers around LBGs and bright quasars.