The metallicity distribution of HI systems in the EAGLE cosmological simulation

Abstract: The metallicity of strong HI systems, spanning from damped Lyman-alpha absorbers (DLAs) to Lyman-limit systems (LLSs) is explored between z = 5->0 using the EAGLE high-resolution cosmological hydrodynamic simulation of galaxy formation. The metallicities of LLSs and DLAs steadily increase with time in agreement with observations. DLAs are more metal rich than LLSs, although the metallicities in the LLS column density range (NHI = 10^17 -10^20 cm^-2) are relatively flat, evolving from a median HI-weighted metallicity of Z<10^-2 Zsol at z = 3 to ~10^-0.5 Zsol by z = 0. The metal content of HI systems tracks the increasing stellar content of the Universe, holding ~5% of the integrated total metals released from stars at z = 0. We also consider partial LLS (pLLS, NHI = 10^16-10^17 cm^-2) metallicities, and find good agreement with Wotta et al. (2016) for the fraction of systems above (37%) and below (63%) 0.1 Zsol. We also find a large dispersion of pLLS metallicities, although we do not reproduce the observed metallicity bimodality and instead we make the prediction that a larger sample will yield more pLLSs around 0.1 Zsol. We under-predict the median metallicity of strong LLSs, and predict a population of Z < 10^-3 Zsol DLAs at z > 3 that are not observed, which may indicate more widespread early enrichment in the real Universe compared to EAGLE.