Cosmic-Ray Feedback from Supernovae in a Parker-Unstable Medium

TL;DR

This study uses simulations to show that cosmic-ray feedback from supernovae significantly enhances galactic outflows and influences the interstellar medium's structure, with implications for galaxy evolution models.

Contribution

It introduces a comparative simulation approach to quantify cosmic-ray feedback effects in a stratified ISM, highlighting the impact of cosmic rays on outflow dynamics and cloud formation.

Findings

Cosmic-ray injection results in faster, hotter, and more massive outflows.

Cold clouds form via Parker and thermal instabilities, with high mass loading.

Decorrelation of cosmic-ray pressure and gas density reduces cosmic-ray calorimetry.

Abstract

Supernova energy drives interstellar medium (ISM) turbulence and can help launch galactic winds. What difference does it make if $10\%$ of the energy is initially deposited into cosmic rays? To answer this question and study cosmic-ray feedback, we perform galactic patch simulations of a stratified ISM. We compare two magnetohydrodynamic and cosmic ray (MHD+CR) simulations, which are identical except for how each supernova's energy is injected. In one, $10\%$ of the energy is injected as cosmic-ray energy. In the other case, energy injection is strictly thermal and kinetic. We find that cosmic-ray injections drive a faster, hotter, and more massive outflow long after the injections occur. Both simulations show the formation of cold clouds (with a total mass fraction $>50\%$) through the Parker instability and thermal instability. The Parker instability simultaneously produces high mass loading factors $\eta > 10^3$ as it requires few supernovae. We also show how the Parker instability naturally leads to a decorrelation of cosmic-ray pressure and gas density. This decorrelation leads to a significant decrease in the calorimetric fraction for injected cosmic rays, but it depends on having a highly resolved magnetic field.