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Abstract In a seminal paper, Parker showed the vertical stratification of the interstellar medium (ISM) is unstable if magnetic fields and cosmic rays provide too large a fraction of pressure support. Cosmic ray acceleration is linked to star formation, so Parker’s instability and its nonlinear outcomes are a type of star formation feedback. Numerical simulations have shown the instability can significantly restructure the ISM, thinning the thermal gas layer and thickening the magnetic field and cosmic ray layer. However, the timescale on which this occurs is rather long (∼0.4 Gyr). Furthermore, the conditions for instability depend on the model adopted for cosmic ray transport. In this work, we connect the instability and feedback problems by examining the effect of a single, spatially and temporally localized cosmic ray injection on the ISM over ∼1 kpc3scales. We perform cosmic ray magnetohydrodynamic simulations using theAthena++code, varying the background properties, dominant cosmic ray transport mechanism, and injection characteristics between our simulation runs. We find robust effects of buoyancy for all transport models, with disruption of the ISM on timescales as short as 100 Myr when the background equilibrium is dominated by cosmic ray pressure.
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This content will become publicly available on August 28, 2026
Cosmic-Ray Feedback from Supernovae in a Parker-unstable Medium
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 help answer this question and study cosmic-ray feedback, we perform galactic patch simulations of a stratified ISM in a low star formation rate, high magnetic field regime. We compare two magnetohydrodynamic and cosmic-ray 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 factorsη> 103as it does not directly require star formation and 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.
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- Award ID(s):
- 2007323
- PAR ID:
- 10634481
- Publisher / Repository:
- IOP
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 990
- Issue:
- 1
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 75
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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