The Parker Instability in Disk Galaxies

TL;DR

This study uses 3D simulations to analyze the Parker instability in galactic disks, revealing complex structures and potential observational signatures influenced by cosmic rays and magnetic fields.

Contribution

It provides the first detailed 3D simulation analysis of the Parker instability, highlighting multi-modal structures and observational implications related to cosmic rays.

Findings

The instability develops a multi-modal 3D structure.

Growth rates depend on cosmic ray density and magnetic fields.

Synthetic observations suggest rotation measure maps probe cosmic ray content.

Abstract

We examine the evolution of the Parker instability in galactic disks using 3D numerical simulations. We consider a local Cartesian box section of a galactic disk, where gas, magnetic fields and cosmic rays are all initially in a magnetohydrostatic equilibrium. This is done for different choices of initial cosmic ray density and magnetic field. The growth rates and characteristic scales obtained from the models, as well as their dependences on the density of cosmic rays and magnetic fields, are in broad agreement with previous (linearized, ideal) analytical work. However, this non-ideal instability develops a multi-modal 3D structure, which cannot be quantitatively predicted from the earlier linearized studies. This 3D signature of the instability will be of importance in interpreting observations. As a preliminary step towards such interpretations, we calculate synthetic polarized intensity and Faraday rotation measure maps, and the associated structure functions of the latter, from our simulations; these suggest that the correlation scales inferred from rotation measure maps are a possible probe for the cosmic ray content of a given galaxy. Our calculations highlight the importance of cosmic rays in these measures, making them an essential ingredient of realistic models of the interstellar medium.