# NIHAO XIX: How supernova feedback shapes the galaxy baryon cycle

## TL;DR

This paper uses NIHAO simulations to analyze how supernova feedback influences the galaxy baryon cycle, regulating star formation, driving outflows, and affecting galaxy mass and baryon content across different scales.

## Contribution

It provides a detailed characterization of supernova feedback effects on galaxy evolution, including outflow rates, gas recycling, and the impact on baryon content, based on high-resolution simulations.

## Key findings

- SN feedback prevents HI condensation into H2, reducing star formation in dwarfs.
- Most outflowing gas is cold and falls back, creating a fountain cycle that reduces galaxy mass.
- Galactic winds divert inflowing gas, explaining low baryon content in ultradwarves.

## Abstract

We have used the NIHAO 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 $v_{\rm vir}\simeq 67{\rm\,km\,s}^{-1}$, the outflow rate follows the relation: $\dot{M}_{\rm out}=23\,(v_{\rm vir}/67{\rm\,km\,s}^{-1})^{-4.6}\,{\rm SFR}$. $20\%$ to $70\%$ 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 onto the galaxy on a $\sim 1\,$Gyr timescale. This `fountain feedback' reduces the masses of galaxies by a factor of two to four, since gas spends half to three quarter of its time in the fountain. Less than $10\%$ of the ejected gas mixes with the hot circumgalactic medium and this gas is usually not reaccreted. On scales as large as $6r_{\rm vir}$, galactic winds divert the incoming gas from cosmic filaments and prevent if from accreting onto galaxies (pre-emptive feedback). This process is the main reason for the low baryon content of ultradwarves.