NIHAO XIX: how supernova feedback shapes the galaxy baryon cycle

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 H-2 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) greater than or similar to 67 km s(-1), the outflow rate follows the relation: (M)over dot(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 similar to 1Gyr 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.