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 approximate to 10(9)-6 x 10(12) M-circle dot at z = 2, including 12 haloes in the mass range M-h similar to 10(11)-10(12) M-circle dot 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 HI 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 HI covering fractions for Lyman limit systems (LLSs) consistent with measurements around z similar to 2-2.5 LBGs; these covering fractions are a factor similar to 2 higher than our previous calculations without galactic winds. The fractions of HI absorbers arising in inflows and in outflows are on average similar to 50 per cent but exhibit significant time variability, ranging from similar to 10 to similar to 90 per cent. For our most massive haloes, we find a factor similar to 3 deficit in the LLS covering fraction relative to what is measured around quasars at z similar to 2, suggesting that the presence of a quasar may affect the properties of halo gas on similar to 100 kpc scales. The predicted covering fractions, which decrease with time, peak at M-h similar to 10(11)-10(12) M-circle dot, 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.