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

TL;DR

This study uses the EAGLE simulation to analyze the distribution of neutral hydrogen around high-redshift galaxies and quasars, successfully matching observed HI covering fractions and revealing how feedback influences HI distribution.

Contribution

It demonstrates that the EAGLE simulation accurately reproduces observed HI distributions and provides new fitting functions for HI covering fractions around massive halos.

Findings

HI covering fractions increase with redshift

Distribution of HI is weakly dependent on halo mass

Simulation matches observed HI column density and covering fractions

Abstract

The observed high covering fractions of neutral hydrogen (HI) with column densities above $\sim 10^{17} \rm{cm}^{-2}$ 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 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 HI around high-redshift galaxies. We predict the covering fractions of strong HI absorbers ($N_{\rm{HI}} \gtrsim 10^{17} \rm{cm}^{-2}$) inside haloes to increase rapidly with redshift but to depend only weakly on halo mass. For massive ($M_{200} \gtrsim 10^{12} {\rm M_{\odot}}$) halos 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 HI covering fractions to the high observed values, the distribution of strong absorbers in and around halos of a fixed mass is insensitive to factor of two variations in the strength of the stellar feedback. In contrast, at fixed stellar mass the predicted HI distribution is highly sensitive to the feedback efficiency. The fiducial EAGLE simulation reproduces both the observed global column density distribution function of HI and the observed radial covering fraction profiles of strong HI absorbers around LBGs and bright quasars.