NIHAO XIX: how supernova feedback shapes the galaxy baryon cycle

Abstract : We have used the NIHAO (Numerical Investigation of a Hundred Astrophysical Objects) simulations to explore how supernovae (SNe) affect star formation in galaxies. We find that SN feedback operates on all scales from the interstellar medium (ISM) to several virial radii. SNe regulate star formation by preventing condensation of HI into H₂ and by moving cold neutral gas to the hot HII phase. The first effect explains why the cold neutral gas in dwarf galaxies forms stars inefficiently. The second maintains the hot ISM of massive galaxies (HII vents out at lower masses). At v_vir ≳ 67 km s^{-1}, the outflow rate follows the relation: \dot{M}_out=23 (v_vir/67 km s^{-1})^{-4.6} SFR. 20{{ per cent}}-70{{ per cent}} of the gas expelled from galaxies escapes from the halo (ejective feedback) but outflows are dominated by cold swept-up gas, most of which falls back on to the galaxy on a {∼ } 1 Gyr time-scale. This `fountain feedback' reduces the masses of galaxies by a factor of 2-4, since gas spends half to three quarter of its time in the fountain. Less than 10{{ per cent}} of the ejected gas mixes with the hot circumgalactic medium and this gas is usually not reaccreted. On scales as large as 6r_vir, galactic winds divert the incoming gas from cosmic filaments and prevent it from accreting on to galaxies (pre-emptive feedback). This process is the main reason for the low baryon content of ultradwarves.