Neutral hydrogen in galaxy haloes at the peak of the cosmic star formation history

We use high-resolution cosmological zoom-in simulations from the FIRE (Feedback in Realistic Environments) project to make predictions for the covering fractions of neutral hydrogen around galaxies at z = 2–4. These simulations resolve the interstellar medium of galaxies and explicitly implement a comprehensive set of stellar feedback mechanisms. Our simulation sample consists of 16 main haloes covering the mass range M_h ≈ 10^9–6 × 10^(12) M_⊙ at z = 2, including 12 haloes in the mass range M_h ∼ 10^(11)–10^(12) M_⊙ corresponding to Lyman break galaxies (LBGs). We process our simulations with a ray tracing method to compute the ionization state of the gas. Galactic winds increase the H i covering fractions in galaxy haloes by direct ejection of cool gas from galaxies and through interactions with gas inflowing from the intergalactic medium. Our simulations predict H i covering fractions for Lyman limit systems (LLSs) consistent with measurements around z ∼ 2–2.5 LBGs; these covering fractions are a factor ∼2 higher than our previous calculations without galactic winds. The fractions of H i absorbers arising in inflows and in outflows are on average ∼50 per cent but exhibit significant time variability, ranging from ∼10 to ∼90 per cent. For our most massive haloes, we find a factor ∼3 deficit in the LLS covering fraction relative to what is measured around quasars at z ∼ 2, suggesting that the presence of a quasar may affect the properties of halo gas on ∼100 kpc scales. The predicted covering fractions, which decrease with time, peak at M_h ∼ 10^(11)–10^(12) M_⊙, near the peak of the star formation efficiency in dark matter haloes. In our simulations, star formation and galactic outflows are highly time dependent; H i covering fractions are also time variable but less so because they represent averages over large areas.

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