The Role of the Parker Instability in Structuring the Interstellar Medium

TL;DR

This study investigates how cosmic ray transport mechanisms influence the Parker instability, revealing their critical role in shaping the interstellar medium and galaxy evolution through nonlinear dynamics and phase structure modifications.

Contribution

It expands previous models by including radiative cooling, a realistic gravitational potential, and nonlinear simulations, highlighting cosmic ray transport as a key driver of the Parker instability.

Findings

Cosmic ray transport away from compression points drives the instability.

Streaming cosmic rays heat and inflate diffuse gas, altering phase structure.

Short wavelength modes dominate growth in 3D, affecting observable Faraday rotation maps.

Abstract

The Parker instability, a Rayleigh-Taylor like instability of thermal gas supported against gravity by magnetic fields and cosmic rays, is thought to be dynamically important for galaxy evolution, possibly promoting molecular cloud formation and the galactic dynamo. In previous work, we examined the effect of three different cosmic ray transport models on the Parker instability: decoupled ($\gamma_c = 0$), locked to the thermal gas ($\gamma_c = 4/3$) and coupled to the gas with streaming by self-confinement. We expand upon that work here by considering radiative cooling, a smooth gravitational potential, and simulations into the nonlinear regime. We determine that cosmic ray transport away from compression points, whether by diffusion or streaming, is the largest driver of the instability. Heating due to cosmic ray streaming is also destabilizing and especially affects the nonlinear regime. While cooling de-pressurizes the dense gas, streaming cosmic rays heat and inflate the diffuse extraplanar gas, greatly modifying the phase structure of the medium. In 3D, we find that the fastest growth favors short wavelength modes in the horizontal direction perpendicular to the background magnetic field; this is imprinted on Faraday rotation measure maps that may be used to detect the Parker instability. The modifications to the Parker instability that we observe in this work have large implications for the structure and evolution of galaxies, and they highlight the major role that cosmic rays play in shaping their environments.