The Impact of Cosmic Ray Injection on Magnetic Flux Tubes in a Galactic Disk

TL;DR

This study investigates how localized cosmic ray injections influence magnetic flux tubes and the interstellar medium in galactic disks, revealing that injection effects vary significantly with the dominant cosmic ray transport mechanism.

Contribution

The paper introduces the first large-scale cosmic ray magnetohydrodynamic simulations examining the impact of localized cosmic ray injection on galactic disk stability.

Findings

Injection disrupts the interstellar medium faster than Parker instability.

Transport by advection leads to rapid disruption (<100 Myr).

Transport by diffusion causes overpressure and buoyant rise of flux tubes.

Abstract

In galactic disks, the Parker instability results when non-thermal pressure support exceeds a certain threshold. The non-thermal pressures considered in the Parker instability are cosmic ray pressure and magnetic pressure. This instability takes a long time to saturate $(>500 \, \mathrm{Myr})$ and assumes a background with fixed cosmic ray pressure to gas pressure ratio. In reality, galactic cosmic rays are injected into localized regions $(< 100 \,\mathrm{pc})$ by events like supernovae, increasing the cosmic ray pressure to gas pressure ratio. In this work, we examine the effect of such cosmic ray injection on large scales $ (\sim 1\,\mathrm{kpc})$ in cosmic ray magnetohydrodynamic simulations using the \texttt{Athena++} code. We vary the background properties, dominant cosmic ray transport mechanism, and injection characteristics between our simulation runs. We find the injection will disrupt the interstellar medium on shorter timescales than the Parker instability. If cosmic ray transport by advection is dominant, cosmic ray injection disrupts the disk on short time scales $(<100\,\mathrm{Myr})$. If cosmic ray transport by the streaming instability is dominant, the injection creates a buoyant flux tube long after the initial injection $(>150\,\mathrm{Myr})$. Finally, when cosmic ray transport by diffusion dominates, the injected cosmic rays make an entire flux tube over pressured in a short time $(\sim 10 \, \mathrm{Myr})$. This over pressure pushes gas off the tube and drives buoyant rise on time scales similar to the advection dominated case.